format-patch: --attach/inline uses filename instead of SHA1

format-patch: --attach/inline uses filename instead of SHA1

Currently when format-patch is used with --attach or --inline the patch
attachment has the SHA1 of the commit for its filename.  This replaces
the SHA1 with the filename used by format-patch when outputting to
files.

Fix tests relying on the SHA1 output and add a test showing how the
--suffix option affects the attachment filename output.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>