object-file: handle compat objects in check_object_signature

Update check_object_signature to find the hash algorithm the exising
signature uses, and to use the same hash algorithm when recomputing it
to check the signature is valid.

This will be useful when teaching git ls-tree to display objects
encoded with the compat hash algorithm.

Signed-off-by: "Eric W. Biederman" <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ebiederm

object-file.c

@@ -1094,9 +1094,11 @@ int check_object_signature(struct repository *r, const struct object_id *oid,
 			   void *buf, unsigned long size,
 			   enum object_type type)
 {
+	const struct git_hash_algo *algo =
+		oid->algo ? &hash_algos[oid->algo] : r->hash_algo;
 	struct object_id real_oid;
 
-	hash_object_file(r->hash_algo, buf, size, type, &real_oid);
+	hash_object_file(algo, buf, size, type, &real_oid);
 
 	return !oideq(oid, &real_oid) ? -1 : 0;
 }