Merge commit template

Description

With the current merge request system, little attention is paid to the merge commit message apart from references to things like issues or other merge requests. However, the merge commit could expose much more information about the role of the merge request and/or the branches that are being merged.

Currently, you can create templates for merge requests in order to enforce good practices. This can be used to decorate the commit message by including the description but this is more or less a "hack".

Proposal

Allow the creation of merge commit template(s), just like you can create markdowns and save them in .gitlab/merge_request_templates. As the commit message has information related to the merge request, a format like handlebars may be more appropriate to allow syntaxes such as Merge branch {{MERGE_BRANCH}} into {{MERGE_TARGET}}.