cache: add a function to read an OID of a specific algorithm

Currently, we always read a object ID of the current algorithm with
oidread.  However, once we start converting objects, we'll need to
consider what happens when we want to read an object ID of a specific
algorithm, such as the compatibility algorithm.  To make this easier,
let's define oidread_algop, which specifies which algorithm we should
use for our object ID, and define oidread in terms of it.

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

Author: bk2204

hash.h

@@ -73,10 +73,15 @@ static inline void oidclr(struct object_id *oid)
 	oid->algo = hash_algo_by_ptr(the_hash_algo);
 }
 
+static inline void oidread_algop(struct object_id *oid, const unsigned char *hash, const struct git_hash_algo *algop)
+{
+	memcpy(oid->hash, hash, algop->rawsz);
+	oid->algo = hash_algo_by_ptr(algop);
+}
+
 static inline void oidread(struct object_id *oid, const unsigned char *hash)
 {
-	memcpy(oid->hash, hash, the_hash_algo->rawsz);
-	oid->algo = hash_algo_by_ptr(the_hash_algo);
+	oidread_algop(oid, hash, the_hash_algo);
 }
 
 static inline int is_empty_blob_sha1(const unsigned char *sha1)