[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.

Some details

MergeRequest currently has a merge_commit_message method that is used if no commit_message is provided to the merge endpoint. This method is also exposed to the frontend through the MergeRequest entity. We can add a squash_commit_message method on MergeRequest and expose that too. — @DouweM in https://gitlab.com/gitlab-org/gitlab-ce/issues/47149#note_128929565