does not support git worktrees

Environment:
- Atom 1.7.2
- Package: open-on-github 1.0.1
- OS x el capitan

I was able to use this successfully on a regular clone and one created with `--separate-git-dir` (I like keeping my .git dir in a separate location). However, using git's new-ish feature, worktrees does not seem to work with open-on-github.

I get this error:

```
No repository found for path: /path/to/file.
```
