* 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