Bypass push rules for merge to ref service
What does this MR do?
Turns out push rules to validate commit message does
not apply in the context of automatic merge to
refs/merge-requests/:iid/merge. Mainly because
if it fails to merge to it, we currently can't give enough
preemptive feedback to the user and it'll turn the
merge request unmergeable (given we automatically
mark it as unmergeable if we can't merge to the ref - it's attached to the MergeRequest#merge_status).
In general, it's a systematic operation, which already bypasses git hooks.
This was recognized through 500's at https://gitlab.com/gitlab-org/gitlab-ce/issues/64184. Which was trying to
validate a nil commit message (through params[:commit_message]) when a push rule was set. Though, we already
fallback to the default_commit_message if no commit_message param is given.
We currently disabled the feature flag of it to avoid further 500's (merge_ref_auto_sync).
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64184