fix: add some comments around non-obvious sparse checkout branches of run-task - #1043

[taskcluster 2026-09-16T17:03:49.298Z] Worker Type (mozilla-t/pre-commit) settings:
[taskcluster 2026-09-16T17:03:49.298Z]   {
[taskcluster 2026-09-16T17:03:49.298Z]     "generic-worker": {
[taskcluster 2026-09-16T17:03:49.298Z]       "config": {
[taskcluster 2026-09-16T17:03:49.298Z]         "capacity": 1,
[taskcluster 2026-09-16T17:03:49.298Z]         "headlessTasks": true
[taskcluster 2026-09-16T17:03:49.298Z]       },
[taskcluster 2026-09-16T17:03:49.298Z]       "engine": "multiuser",
[taskcluster 2026-09-16T17:03:49.298Z]       "go-arch": "amd64",
[taskcluster 2026-09-16T17:03:49.298Z]       "go-os": "linux",
[taskcluster 2026-09-16T17:03:49.298Z]       "go-version": "go1.27.1",
[taskcluster 2026-09-16T17:03:49.298Z]       "release": "https://github.com/taskcluster/taskcluster/releases/tag/v108.1.0",
[taskcluster 2026-09-16T17:03:49.298Z]       "revision": "f86624a8bf3761cdbc48de2c7f74bae44d7464dd",
[taskcluster 2026-09-16T17:03:49.298Z]       "source": "https://github.com/taskcluster/taskcluster/commits/f86624a8bf3761cdbc48de2c7f74bae44d7464dd",
[taskcluster 2026-09-16T17:03:49.298Z]       "version": "108.1.0"
[taskcluster 2026-09-16T17:03:49.298Z]     },
[taskcluster 2026-09-16T17:03:49.298Z]     "image": "projects/taskcluster-imaging/global/images/gw-fxci-gcp-l1-2404-amd64-headless-googlecompute-2026-09-09",
[taskcluster 2026-09-16T17:03:49.298Z]     "instance-id": "1644776944707057262",
[taskcluster 2026-09-16T17:03:49.298Z]     "instance-type": "projects/887720501152/machineTypes/c3d-standard-4",
[taskcluster 2026-09-16T17:03:49.298Z]     "local-ipv4": "10.138.0.177",
[taskcluster 2026-09-16T17:03:49.298Z]     "project-id": "fxci-production-level1-workers",
[taskcluster 2026-09-16T17:03:49.298Z]     "public-hostname": "mozilla-t-pre-commit-gzsc9m0cscm3td5ps1ah4a.c.fxci-production-level1-workers.internal",
[taskcluster 2026-09-16T17:03:49.298Z]     "public-ipv4": "104.198.110.144",
[taskcluster 2026-09-16T17:03:49.298Z]     "region": "us-west1",
[taskcluster 2026-09-16T17:03:49.298Z]     "zone": "us-west1-b"
[taskcluster 2026-09-16T17:03:49.298Z]   }
[taskcluster 2026-09-16T17:03:49.298Z] Task ID: aC75bIR9Qw6RguAeUE6LDw
[taskcluster 2026-09-16T17:03:49.298Z] === Task Starting ===
[taskcluster 2026-09-16T17:03:49.976Z] [mounts] Moving existing writable directory cache lint-pre-commit-uv-v1 from /home/generic-worker/caches/ZISoWXGFQ5eJLPVHSLxVrA to /home/task_aC75bIR9Qw6Rg_0/cache0
[taskcluster 2026-09-16T17:03:49.976Z] [mounts] Creating directory /home/task_aC75bIR9Qw6Rg_0
[taskcluster 2026-09-16T17:03:49.989Z] [mounts] Successfully mounted writable directory cache '/home/task_aC75bIR9Qw6Rg_0/cache0'
[taskcluster 2026-09-16T17:03:49.989Z] [mounts] Moving existing writable directory cache lint-pre-commit-repos-v1 from /home/generic-worker/caches/d_KXpM1jSj-wq40WeZa82Q to /home/task_aC75bIR9Qw6Rg_0/cache1
[taskcluster 2026-09-16T17:03:49.989Z] [mounts] Creating directory /home/task_aC75bIR9Qw6Rg_0
[taskcluster 2026-09-16T17:03:50.001Z] [mounts] Successfully mounted writable directory cache '/home/task_aC75bIR9Qw6Rg_0/cache1'
[taskcluster 2026-09-16T17:03:50.001Z] [mounts] Moving existing writable directory cache lint-pre-commit-checkout-taskcluster-taskgraph-v1 from /home/generic-worker/caches/MuqS-mSMTVCQpYHqAI1Tgw to /home/task_aC75bIR9Qw6Rg_0/cache2
[taskcluster 2026-09-16T17:03:50.001Z] [mounts] Creating directory /home/task_aC75bIR9Qw6Rg_0
[taskcluster 2026-09-16T17:03:50.015Z] [mounts] Successfully mounted writable directory cache '/home/task_aC75bIR9Qw6Rg_0/cache2'
[taskcluster 2026-09-16T17:03:50.015Z] [d2g] Using cached docker image "docker.io/mozillareleases/taskgraph:run-task-v24.1.1@sha256:9b9a1a3ca42025bb8d7eba19e1b8759f04504097265074df25bbc997bd802543"
[taskcluster 2026-09-16T17:03:50.019Z] Executing command 0: docker run -t --name taskcontainer_I-AtMGWIRk-Rz51M50yrQA --memory-swap -1 --pids-limit -1 --pull=never --log-driver=none '--add-host=localhost.localdomain:127.0.0.1' -v '/home/task_aC75bIR9Qw6Rg_0/cache0:/builds/worker/.cache/uv' -v '/home/task_aC75bIR9Qw6Rg_0/cache1:/builds/worker/.cache/pre-commit' -v /home/task_aC75bIR9Qw6Rg_0/cache2:/builds/worker/checkouts --network gw-task-aC75bIR9Qw6R-0 '--add-host=taskcluster:172.18.0.1' --env-file 'env.list' -- 'mozillareleases/taskgraph:run-task-v24.1.1@sha256:9b9a1a3ca42025bb8d7eba19e1b8759f04504097265074df25bbc997bd802543' run-task --src-checkout=/builds/worker/checkouts/src --task-cwd=/builds/worker/checkouts/src -- bash -c 'uv tool install pre-commit && /builds/worker/.local/bin/pre-commit run --all-files'
[setup 2026-09-16T17:03:50.645+00:00] run-task started in /
[setup 2026-09-16T17:03:50.645+00:00] Invoked by command: --src-checkout=/builds/worker/checkouts/src --task-cwd=/builds/worker/checkouts/src -- bash -c uv tool install pre-commit && /builds/worker/.local/bin/pre-commit run --all-files
[setup 2026-09-16T17:03:50.645+00:00] Python version: 3.13.5
[setup 2026-09-16T17:03:50.645+00:00] Subprocess python version: 
cpython-3.13.5-linux-x86_64-gnu    /usr/bin/python3.13
cpython-3.13.5-linux-x86_64-gnu    /usr/bin/python3 -> python3.13
[cache 2026-09-16T17:03:50.739+00:00] cache /builds/worker/.cache/uv exists; requirements: gid=1000 uid=1000 version=1
[cache 2026-09-16T17:03:50.739+00:00] cache /builds/worker/.cache/pre-commit exists; requirements: gid=1000 uid=1000 version=1
[cache 2026-09-16T17:03:50.739+00:00] cache /builds/worker/checkouts exists; requirements: gid=1000 uid=1000 version=1
[setup 2026-09-16T17:03:50.740+00:00] running as worker:worker
[vcs 2026-09-16T17:03:50.740+00:00] executing ['git', 'config', '--global', '--add', 'safe.directory', '/builds/worker/checkouts/src']
[vcs 2026-09-16T17:03:50.742+00:00] executing ['git', 'fetch', 'https://github.com/bhearsum/taskgraph', 'fix-add-some-comments-around-n']
[vcs 2026-09-16T17:03:51.244+00:00] From https://github.com/bhearsum/taskgraph
[vcs 2026-09-16T17:03:51.244+00:00]  * branch            fix-add-some-comments-around-n -> FETCH_HEAD
[vcs 2026-09-16T17:03:51.246+00:00] executing ['git', 'checkout', '-f', '-B', 'fix-add-some-comments-around-n', 'f0a5394d63c5d04e795ae66023d8e4909c3a4fb8']
[vcs 2026-09-16T17:03:51.252+00:00] Reset branch 'fix-add-some-comments-around-n'
[vcs 2026-09-16T17:03:51.252+00:00] cleaning git checkout...
[vcs 2026-09-16T17:03:51.252+00:00] executing ['git', 'clean', '-nxdff']
[vcs 2026-09-16T17:03:51.255+00:00] removing ['/builds/worker/checkouts/src/.ruff_cache/']
[vcs 2026-09-16T17:03:51.257+00:00] successfully cleaned git checkout!
[vcs 2026-09-16T17:03:51.258+00:00] TinderboxPrint:<a href='https://github.com/bhearsum/taskgraph/commit/f0a5394d63c5d04e795ae66023d8e4909c3a4fb8' title='Built from taskgraph commit f0a5394d63c5d04e795ae66023d8e4909c3a4fb8'>f0a5394d63c5d04e795ae66023d8e4909c3a4fb8</a>
[setup 2026-09-16T17:03:51.258+00:00] UV_CACHE_DIR is /builds/worker/.cache/uv
[task 2026-09-16T17:03:51.258+00:00] executing ['bash', '-c', 'uv tool install pre-commit && /builds/worker/.local/bin/pre-commit run --all-files']
[task 2026-09-16T17:03:51.321+00:00] Resolved 10 packages in 52ms
[task 2026-09-16T17:03:51.326+00:00] warning: Failed to hardlink files; falling back to full copy. This may lead to degraded performance.
[task 2026-09-16T17:03:51.326+00:00]          If the cache and target directories are on different filesystems, hardlinking may not be supported.
[task 2026-09-16T17:03:51.326+00:00]          If this is intentional, set `export UV_LINK_MODE=copy` or use `--link-mode=copy` to suppress this warning.
[task 2026-09-16T17:03:51.339+00:00] Installed 10 packages in 16ms
[task 2026-09-16T17:03:51.339+00:00]  + cfgv==3.5.0
[task 2026-09-16T17:03:51.340+00:00]  + distlib==0.4.3
[task 2026-09-16T17:03:51.340+00:00]  + filelock==3.32.7
[task 2026-09-16T17:03:51.340+00:00]  + identify==2.6.19
[task 2026-09-16T17:03:51.340+00:00]  + nodeenv==1.10.0
[task 2026-09-16T17:03:51.340+00:00]  + platformdirs==4.11.9
[task 2026-09-16T17:03:51.340+00:00]  + pre-commit==4.6.2
[task 2026-09-16T17:03:51.340+00:00]  + python-discovery==1.6.0
[task 2026-09-16T17:03:51.340+00:00]  + pyyaml==6.0.3
[task 2026-09-16T17:03:51.340+00:00]  + virtualenv==21.7.10
[task 2026-09-16T17:03:51.341+00:00] Installed 1 executable: pre-commit
[task 2026-09-16T17:03:51.341+00:00] warning: `/builds/worker/.local/bin` is not on your PATH. To use installed tools, run `export PATH="/builds/worker/.local/bin:$PATH"` or `uv tool update-shell`.
[task 2026-09-16T17:03:51.654+00:00] trim trailing whitespace.................................................Passed
[task 2026-09-16T17:03:51.702+00:00] fix end of files.........................................................Passed
[task 2026-09-16T17:03:51.816+00:00] check yaml...............................................................Passed
[task 2026-09-16T17:03:51.869+00:00] check for added large files..............................................Passed
[task 2026-09-16T17:03:51.899+00:00] ruff (legacy alias)......................................................Passed
[task 2026-09-16T17:03:51.930+00:00] ruff format..............................................................Passed
[task 2026-09-16T17:03:51.949+00:00] uv-lock..................................................................Passed
[task 2026-09-16T17:03:52.143+00:00] yamllint.................................................................Passed
[task 2026-09-16T17:03:52.464+00:00] codespell................................................................Passed
[taskcluster 2026-09-16T17:03:52.941Z]                        Exit Code: 0
[taskcluster 2026-09-16T17:03:52.941Z]                        User Time: 10.202ms
[taskcluster 2026-09-16T17:03:52.941Z]                      Kernel Time: 17.344ms
[taskcluster 2026-09-16T17:03:52.941Z]                        Wall Time: 2.922397187s
[taskcluster 2026-09-16T17:03:52.941Z]  Average Available System Memory: 14.46 GiB
[taskcluster 2026-09-16T17:03:52.941Z]       Average System Memory Used: 1.15 GiB
[taskcluster 2026-09-16T17:03:52.941Z]          Peak System Memory Used: 1.26 GiB
[taskcluster 2026-09-16T17:03:52.941Z]              Total System Memory: 15.61 GiB
[taskcluster 2026-09-16T17:03:52.941Z]                           Result: SUCCEEDED
[taskcluster 2026-09-16T17:03:52.941Z] === Task Finished ===
[taskcluster 2026-09-16T17:03:52.941Z] Task Duration: 2.922899797s
[taskcluster 2026-09-16T17:03:52.983Z] [mounts] Preserving cache: Moving "/home/task_aC75bIR9Qw6Rg_0/cache0" to "/home/generic-worker/caches/ZISoWXGFQ5eJLPVHSLxVrA"
[taskcluster 2026-09-16T17:03:52.984Z] [mounts] Preserving cache: Moving "/home/task_aC75bIR9Qw6Rg_0/cache1" to "/home/generic-worker/caches/d_KXpM1jSj-wq40WeZa82Q"
[taskcluster 2026-09-16T17:03:52.984Z] [mounts] Preserving cache: Moving "/home/task_aC75bIR9Qw6Rg_0/cache2" to "/home/generic-worker/caches/MuqS-mSMTVCQpYHqAI1Tgw"