midx: do not try to look at every object in the index

Similar to previous issues around doing way too much verification at loading
time, checking whether the object index chunk is monotonic is better left for
git-fsck instead of every single time we want to look at something in the
repository.

As midx files grow, this starts taking more and more times. As an example, I
went looking for this because it's taking about 1.5s to do a single object
lookup in a repository that's ended up with a 7G multi-pack-index file.

Author: carlosmn

src/libgit2/midx.c

@@ -114,8 +114,6 @@ static int midx_parse_oid_lookup(
 		const unsigned char *data,
 		struct git_midx_chunk *chunk_oid_lookup)
 {
-	uint32_t i;
-	unsigned char *oid, *prev_oid, zero_oid[GIT_OID_MAX_SIZE] = {0};
 	size_t oid_size = git_oid_size(idx->oid_type);
 
 	if (chunk_oid_lookup->offset == 0)
@@ -125,13 +123,7 @@ static int midx_parse_oid_lookup(
 	if (chunk_oid_lookup->length != idx->num_objects * oid_size)
 		return midx_error("OID Lookup chunk has wrong length");
 
-	idx->oid_lookup = oid = (unsigned char *)(data + chunk_oid_lookup->offset);
-	prev_oid = zero_oid;
-	for (i = 0; i < idx->num_objects; ++i, oid += oid_size) {
-		if (git_oid_raw_cmp(prev_oid, oid, oid_size) >= 0)
-			return midx_error("OID Lookup index is non-monotonic");
-		prev_oid = oid;
-	}
+	idx->oid_lookup = (unsigned char *)(data + chunk_oid_lookup->offset);
 
 	return 0;
 }