Git hooks don't fire when committing from the UI

Summary

Git hooks no longer fire when committing from the web UI

Steps to reproduce

1. Add a commit message hook to a project. For example, try foo.*.
2. Then, try to commit a file in the UI with a commit message like test.
3. Observe that the file is committed successfully.

Expected behavior

On 8.6 and earlier, these same steps produced a pre-receive hook error.

cc/ @DouweM