fmt-patch: Support --attach

fmt-patch: Support --attach

This patch touches a couple of files, because it adds options to print a
custom text just after the subject of a commit, and just after the
diffstat.

[jc: made "many dashes" used as the boundary leader into a single
 variable, to reduce the possibility of later tweaks to miscount the
 number of dashes to break it.]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>