Improve the logic of the file entry Git hooks to support more cases

Improve the logic of the file entry Git hooks to support more cases

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.