Add a mention of the merge request into its default commit message.
Merge branch 'reference_merge_request_in_commit_message' into 'master'

Add reference to the merge request in the default merge request commit message

This adds a link to the merge request to the merge request commit.

If merge requests are used for code review, the merge in git is now linked to the code review.

See http://feedback.gitlab.com/forums/176466-general/suggestions/6088000-in-the-default-merge-request-commit-message-add-a