git-crypt used alongside Github for Mac reliably breaks

This is essentially Issue #26 for [Github for Mac](https://mac.github.com/) instead of Atlassian SourceTree.

If I have a file to be encrypted upon commit, `git commit ...` from the command line works as expected. Clicking commit from the Github app produces the following message.

```
[master 3846a0e] MY_COMMIT_MESSAGE

"git-crypt" clean: git-crypt: command not found
error: cannot feed the input to external filter "git-crypt" clean
error: external filter "git-crypt" clean failed -1
error: external filter "git-crypt" clean failed
fatal: FILE_TO_BE_ENCRYPTED: clean filter 'git-crypt' failed
(128)
```

The file(s) then pass through to the repository unencrypted.

I understand this might not be a high priority. Just thought it might as well be documented. :information_source: