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>

convert-objects.c

@@ -39,7 +39,7 @@ static struct entry *lookup_entry(unsigned char *sha1)
 	while (low < high) {
 		int next = (low + high) / 2;
 		struct entry *n = convert[next];
-		int cmp = memcmp(sha1, n->old_sha1, 20);
+		int cmp = hashcmp(sha1, n->old_sha1);
 		if (!cmp)
 			return n;
 		if (cmp < 0) {