Can't commit with git-secrets

## Description

Git-secrets fails to run (I think because it can't find git-sh-setup).
Specifically, the error raised points to [line 46](https://github.com/awslabs/git-secrets/blob/c11c229a8eebb3545e5fd3bab4e8aa55a7f19383/git-secrets#L46), and the built-in git has a weird exec-path:
```bash
$ /Applications/GitHub\ Desktop.app/Contents/Resources/app/git/bin/git --exec-path
//libexec/git-core
```

## Version

* GitHub Desktop: 1.6.0

* Operating system: macOS 10.13.6 (17G4015)

## Steps to Reproduce

Set up [git-secrets](https://github.com/awslabs/git-secrets) in a repo, then try to commit any file.

### Expected Behavior

Commit succeeds.

### Actual Behavior

Commit fails with error `No such file or directory`.

## Additional Information

### Logs

```
2019-01-18T22:18:28.902Z - error: [ui] `git commit -F -` exited with an unexpected code: 1.
/usr/local/bin/git-secrets
/usr/local/bin/git-secrets: line 46: /Applications/GitHub: No such file or directory
```