Merge branch 'tc/merge-m-log' into maint

* tc/merge-m-log: merge: --log appends shortlog to message if specified fmt-merge-msg: add function to append shortlog only fmt-merge-msg: refactor merge title formatting fmt-merge-msg: minor refactor of fmt_merge_msg() merge: rename variable merge: update comment t7604-merge-custom-message: show that --log doesn't append to -m t7604-merge-custom-message: shift expected output creation