Merge branch 'bc/hash-transition-interop-part-1'

SHA-256 transition.

* bc/hash-transition-interop-part-1:
  hex: print objects using the hash algorithm member
  hex: default to the_hash_algo on zero algorithm value
  builtin/pack-objects: avoid using struct object_id for pack hash
  commit-graph: don't store file hashes as struct object_id
  builtin/show-index: set the algorithm for object IDs
  hash: provide per-algorithm null OIDs
  hash: set, copy, and use algo field in struct object_id
  builtin/pack-redundant: avoid casting buffers to struct object_id
  Use the final_oid_fn to finalize hashing of object IDs
  hash: add a function to finalize object IDs
  http-push: set algorithm when reading object ID
  Always use oidread to read into struct object_id
  hash: add an algo member to struct object_id

branch.c

@@ -322,7 +322,7 @@ void create_branch(struct repository *r,
 		transaction = ref_transaction_begin(&err);
 		if (!transaction ||
 		    ref_transaction_update(transaction, ref.buf,
-					   &oid, forcing ? NULL : &null_oid,
+					   &oid, forcing ? NULL : null_oid(),
 					   0, msg, &err) ||
 		    ref_transaction_commit(transaction, &err))
 			die("%s", err.buf);