Resolve "gitlab 10.8 assumes that installation is a git working copy."

What does this MR do?

This replaces Gitlab::REVISION with a Gitlab.revision which handles installation that don't have a .git directory. In that case, Gitlab.revision will return Unknown.

Are there points in the code the reviewer needs to double check?

This is a quick fix to resolve the %10.8 regression, but I've suggested a more long-term improvement in https://gitlab.com/gitlab-org/gitlab-ce/issues/46600#note_74743108 where we'd use a new REVISION file and update it at release time.

omnibus-gitlab!2540 (merged) needs to be released in the same release as this fix.

Why was this MR needed?

Because otherwise the revision would be set to fatal: Not a git repository, which isn't very helpful.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #46600 (closed)