Squashing MRs doesn't work on GitLab.com

Reported at https://gitlab.com/gitlab-com/support-forum/issues/1532.

I tried with omnibus-gitlab!1192 (merged), and it works for me locally. I found some error logs in Kibana: https://log.gitlap.com/goto/e59a61283cfe1081af1d3a33389a7ff7

usage: git diff [--no-index] <path> <path>

This is what git diff says when it's not in a repo, which is weird. Additionally, some fail at the git commit step, but I don't see any stderr / stdout in the logs: https://log.gitlap.com/goto/f270a55a0b518ec98dcb8dee37b0c643