Do not use memcmp(sha1_1, sha1_2, 20) with hardcoded length.

Introduces global inline:

	hashcmp(const unsigned char *sha1, const unsigned char *sha2)

Uses memcmp for comparison and returns the result based on the length of
the hash name (a future runtime decision).

Acked-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-commit-tree.c

@@ -69,7 +69,7 @@ static int new_parent(int idx)
 	int i;
 	unsigned char *sha1 = parent_sha1[idx];
 	for (i = 0; i < idx; i++) {
-		if (!memcmp(parent_sha1[i], sha1, 20)) {
+		if (!hashcmp(parent_sha1[i], sha1)) {
 			error("duplicate parent %s ignored", sha1_to_hex(sha1));
 			return 0;
 		}