Avoid git ending with this message:
	"Patch format  is not supported."

With improved error message in the format detection failure case by
Giuseppe Bilotta.
Signed-off-by: NNicolas Sebrecht <ni.s@laposte.net>
Signed-off-by: NBrandon Casey <drafnel@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>