Turns out push rules to validate commit message does
not apply in the context of automatic merge to
the 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).
In general, it's a systematic operation, which already
bypasses user authorization and git hooks.
Therefore, this commit makes it bypass the push rules
at EE as well.
Adds the ground work for writing into
the merge ref refs/merge-requests/:iid/merge the
merge result between source and target branches of
a MR, without further side-effects such as
mailing, MR updates and target branch changes.
