format-patch: use the diff options for the cover letter, too

format-patch: use the diff options for the cover letter, too

Earlier, when you called "git format-patch --cover-letter -M", the
diffstat in the cover letter would not inherit the "-M".  Now it does.

While at it, add a few "|| break" statements in the test's loops;
otherwise, breakages inside the loops would not be caught.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>