Allow Default Merge Request message to be configured
Description
Whenever we Accept a Merge Request, people forget to put the title on the top, which is crucial to get a nice view of the issues in GitExtensions.
Proposal
The default message that comes on top should be configurable on a template. This issue has been discussed before in https://gitlab.com/gitlab-org/gitlab-ce/issues/23462 and there are people not wanting the Merged branch 'this' into 'that' on top.
Edit/Note: I agree that the line is standard for Git, but the standard does not imply that it has to come out on top.
It could also be kept together with the "See merge request !number" in the bottom of the default Merge Request. As it is now it does not make sense to have those two messages spread out in the commit message. Futhermore - when you click on the Merge Request-number you get to a page that says "Request to merge 'this' into 'that'", so keeping only the bottom line "See Merge Request !number" is totally enough, since the top line is redundant (for GitLab).
Links / references
https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/merge_request.rb#L791