Figure out where the path diverges in multiple copies of the same message.

This uses mailinfo to parse multiple message files, and tries
to figure out, starting at the originator, where the path for
multiple messages diverged.  The main use is to figure out
where duplicate mailing list messages are coming from (e.g.,
some subscriber is forwarding messages back to the list).
Merge branch 'master' of https://github.com/fenner/mailinfo