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

sequencer.c

@@ -525,7 +525,7 @@ static int fast_forward_to(struct repository *r,
 	if (!transaction ||
 	    ref_transaction_update(transaction, "HEAD",
 				   to, unborn && !is_rebase_i(opts) ?
-				   &null_oid : from,
+				   null_oid() : from,
 				   0, sb.buf, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		ref_transaction_free(transaction);
@@ -1132,7 +1132,7 @@ int update_head_with_reflog(const struct commit *old_head,
 	transaction = ref_transaction_begin(err);
 	if (!transaction ||
 	    ref_transaction_update(transaction, "HEAD", new_head,
-				   old_head ? &old_head->object.oid : &null_oid,
+				   old_head ? &old_head->object.oid : null_oid(),
 				   0, sb.buf, err) ||
 	    ref_transaction_commit(transaction, err)) {
 		ret = -1;