Sync tests rely on revision from pontoon/.git, and don't have access to it

[bugzilla-to-github]

This issue was created automatically by a script.

Bug 1591995

Bug Reporter: @Pike
CC: @mathjazz

When testing in the docker test environment, git doesn't have access to /app/.git, because it's on a different file system.

That means that some code paths (like writing revisions to the db) aren't tested at all. Perhaps also not at all for hg?

Also, the comment in docker-compose.yml about what depends on .git isn't correct.

This can be hacked around by adding

+            env={'GIT_DISCOVERY_ACROSS_FILESYSTEM':'1'}

in GitRepository.revision, but that's so not safe for prod.