patch-id: take "commit" prefix as well as "diff-tree" prefix

Some time ago we changed git-log in a massive way, and one consequence is
that the keyword changed. Adjust patch-id for that.

[jc: as Linus suggests, allowing both old and new prefix.]
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

patch-id.c

@@ -42,6 +42,8 @@ static void generate_id_list(void)
 
 		if (!memcmp(line, "diff-tree ", 10))
 			p += 10;
+		else if (!memcmp(line, "commit ", 7))
+			p += 7;
 
 		if (!get_sha1_hex(p, n)) {
 			flush_current_id(patchlen, sha1, &ctx);