[Backend] Override squash commit message
Backend Issue for: https://gitlab.com/gitlab-org/gitlab-ce/issues/47149
Please refer to the description of the parent issue for up-to-date information.
This issue was created to track Backend weight independently of Frontend.
MergeRequestcurrently has a
merge_commit_messagemethod that is used if no
commit_messageis provided to the merge endpoint. This method is also exposed to the frontend through the
MergeRequestentity. We can add a
MergeRequestand expose that too. — @DouweM in https://gitlab.com/gitlab-org/gitlab-ce/issues/47149#note_128929565