So it's more clear what could happen. Also add
more tests about the behaviour.

So that whenever we want to reserve more, we're aware,
and don't mess it up.

so that with_repo_branch_commit can properly short-circuit

An upcoming update to rubocop-gitlab-security added additional
violations.

so that we don't have to fetch it for non-forks

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13416#note_37193731

So we just try a cheaper way to detect it if it works or not

Closes gitaly#443

Closes gitaly#433