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-diff.c

@@ -69,7 +69,7 @@ static void stuff_change(struct diff_options *opt,
 	struct diff_filespec *one, *two;
 
 	if (!is_null_sha1(old_sha1) && !is_null_sha1(new_sha1) &&
-	    !memcmp(old_sha1, new_sha1, 20))
+	    !hashcmp(old_sha1, new_sha1))
 		return;
 
 	if (opt->reverse_diff) {