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

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