oid: use memcmp in git_oid__hashcmp

The open-coded version was inherited from git.git. But it
turns out it was based on an older version of glibc, whose
memcmp was not very optimized.

Modern glibc does much better, and some compilers (like gcc
7) can even inline the memcmp into a series of multi-byte
xors.

Upstream is switching to using memcmp in
git/git@0b006014c87f400bd9a86267ed30fd3e7b383884.

src/oid.h

@@ -22,14 +22,7 @@ char *git_oid_allocfmt(const git_oid *id);
 
 GIT_INLINE(int) git_oid__hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
-	int i;
-
-	for (i = 0; i < GIT_OID_RAWSZ; i++, sha1++, sha2++) {
-		if (*sha1 != *sha2)
-			return *sha1 - *sha2;
-	}
-
-	return 0;
+	return memcmp(sha1, sha2, GIT_OID_RAWSZ);
 }
 
 /*