mailpipe: skip `.gitignore` files in Maildir mailboxes.

mailpipe: skip `.gitignore` files in Maildir mailboxes.

I use empty `.gitignore` files so Git will create the
`test/mail-in/new` and `test/mail-in/tmp` directories on checkout, but
the Maildir mailbox thinks they are messages.  Since the files are
*not* messages, skip them when constucting the Maildir message list.

I don't foresee any side effects from this (it's an odd filename for
real Maildir delivery), but I log the skipped filenames just in case.