commit: convert commit_tree* to object_id

Convert the definitions and declarations of commit_tree and
commit_tree_extended to use struct object_id and adjust all usages of
these functions.

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 4610e3d..e5974a5 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1794,8 +1794,9 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		append_merge_tag_headers(parents, &tail);
 	}
 
-	if (commit_tree_extended(sb.buf, sb.len, active_cache_tree->oid.hash,
-			 parents, oid.hash, author_ident.buf, sign_commit, extra)) {
+	if (commit_tree_extended(sb.buf, sb.len, &active_cache_tree->oid,
+				 parents, &oid, author_ident.buf, sign_commit,
+				 extra)) {
 		rollback_index_files();
 		die(_("failed to write commit object"));
 	}