This method was moved to ::Git but it is not cached there which causes performance problems

- `Gitlab::Shell.fetch_remote` now takes a `Gitlab::Git::Repository` instead

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

The list of commits must be generated from the merge request,
not from a diff of the branches.

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

Store the original commit in the commit message. In case of
merge commit, the commits added to the start branch are also
listed.

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