Tentative built-in format-patch.

This only does --stdout right now.  To write into separate files
with pretty-printed filenames like the real thing does, it needs
a bit mroe work.

Signed-off-by: Junio C Hamano <junkio@cox.net>

commit.h

@@ -45,6 +45,7 @@ enum cmit_fmt {
 	CMIT_FMT_FULL,
 	CMIT_FMT_FULLER,
 	CMIT_FMT_ONELINE,
+	CMIT_FMT_EMAIL,
 
 	CMIT_FMT_UNSPECIFIED,
 };