Merge branch 'bc/object-id'

Conversion from uchar[20] to struct object_id continues.

* bc/object-id: (42 commits)
  merge-one-file: compute empty blob object ID
  add--interactive: compute the empty tree value
  Update shell scripts to compute empty tree object ID
  sha1_file: only expose empty object constants through git_hash_algo
  dir: use the_hash_algo for empty blob object ID
  sequencer: use the_hash_algo for empty tree object ID
  cache-tree: use is_empty_tree_oid
  sha1_file: convert cached object code to struct object_id
  builtin/reset: convert use of EMPTY_TREE_SHA1_BIN
  builtin/receive-pack: convert one use of EMPTY_TREE_SHA1_HEX
  wt-status: convert two uses of EMPTY_TREE_SHA1_HEX
  submodule: convert several uses of EMPTY_TREE_SHA1_HEX
  sequencer: convert one use of EMPTY_TREE_SHA1_HEX
  merge: convert empty tree constant to the_hash_algo
  builtin/merge: switch tree functions to use object_id
  builtin/am: convert uses of EMPTY_TREE_SHA1_BIN to the_hash_algo
  sha1-file: add functions for hex empty tree and blob OIDs
  builtin/receive-pack: avoid hard-coded constants for push certs
  diff: specify abbreviation size in terms of the_hash_algo
  upload-pack: replace use of several hard-coded constants
  ...

sequencer.c

@@ -1247,7 +1247,7 @@ static int try_to_commit(struct strbuf *msg, const char *author,
 
 	if (!(flags & ALLOW_EMPTY) && !oidcmp(current_head ?
 					      get_commit_tree_oid(current_head) :
-					      &empty_tree_oid, &tree)) {
+					      the_hash_algo->empty_tree, &tree)) {
 		res = 1; /* run 'git commit' to display error message */
 		goto out;
 	}
@@ -1639,7 +1639,7 @@ static int do_pick_commit(enum todo_command command, struct commit *commit,
 			unborn = 1;
 		} else if (unborn)
 			oidcpy(&head, the_hash_algo->empty_tree);
-		if (index_differs_from(unborn ? EMPTY_TREE_SHA1_HEX : "HEAD",
+		if (index_differs_from(unborn ? empty_tree_oid_hex() : "HEAD",
 				       NULL, 0))
 			return error_dirty_index(opts);
 	}