format-patch: add an option to suppress commit hash

format-patch: add an option to suppress commit hash

Oftentimes, patches created by git format-patch will be stored in
version control or compared with diff.  In these cases, two otherwise
identical patches can have different commit hashes, leading to diff
noise.  Teach git format-patch a --zero-commit option that instead
produces an all-zero hash to avoid this diff noise.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>