format-patch: add --description-file option

format-patch: add --description-file option

This patch makes it possible to directly feed a branch description to
derive the cover letter from. The use case is formatting dynamically
created temporary commits which are not referenced anywhere.

The most obvious alternative would be creating a temporary branch and
setting a description on it, but that doesn't seem particularly elegant.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>