fmt-merge-msg: make attribution into comment lines

fmt-merge-msg: make attribution into comment lines

The submaintainer credit is not something you can compute purely by
looking at the history and its shape, especially in the presense of
fast-forward merges, and this observation makes the information on
the "via" line unreliable.  Let's leave the final determination of
credits up to whoever is making the merge and show them as comments.

Signed-off-by: Junio C Hamano <gitster@pobox.com>