This does two things:
1. We can properly validate log entries in merge commits.
2. We don't check commits that were merged in from other branches.
* build-aux/git-hooks/commit-msg-files.awk (get_commit_changes): Get
the changes compared to the first parent.
* build-aux/git-hooks/pre-push: Only get the first parent of merge
commits when returning the rev-list, and only check "master" or
"emacs-NN" branches.
In addition to starting with a "*", file entries now need a ":"
somewhere in them. This helps reduce false positives with bulleted
lists. Also, support multiple files separated by commas after a "*".
* build-aux/git-hooks/commit-msg-files.awk (check_commit_msg_files):
Accumulate file entries over multiple lines to support the above.
