Merge branch 'ps/maintenance-ref-lock'

"git maintenance" lacked the care "git gc" had to avoid holding
onto the repository lock for too long during packing refs, which
has been remedied.

* ps/maintenance-ref-lock:
  builtin/maintenance: fix locking race when handling "gc" task
  builtin/gc: avoid global state in `gc_before_repack()`
  usage: allow dying without writing an error message
  builtin/maintenance: fix locking race with refs and reflogs tasks
  builtin/maintenance: split into foreground and background tasks
  builtin/maintenance: fix typedef for function pointers
  builtin/maintenance: extract function to run tasks
  builtin/maintenance: stop modifying global array of tasks
  builtin/maintenance: mark "--task=" and "--schedule=" as incompatible
  builtin/maintenance: centralize configuration of explicit tasks
  builtin/gc: drop redundant local variable
  builtin/gc: use designated field initializers for maintenance tasks

Author: gitster

builtin/am.c

@@ -1000,7 +1000,7 @@ static void am_setup(struct am_state *state, enum patch_format patch_format,
 
 	if (!patch_format) {
 		fprintf_ln(stderr, _("Patch format detection failed."));
-		exit(128);
+		die(NULL);
 	}
 
 	if (mkdir(state->dir, 0777) < 0 && errno != EEXIST)
@@ -1178,7 +1178,7 @@ static void NORETURN die_user_resolve(const struct am_state *state)
 		strbuf_release(&sb);
 	}
 
-	exit(128);
+	die(NULL);
 }
 
 /**

builtin/checkout.c

@@ -838,7 +838,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 		init_tree_desc(&trees[0], &tree->object.oid,
 			       tree->buffer, tree->size);
 		if (parse_tree(new_tree) < 0)
-			exit(128);
+			die(NULL);
 		tree = new_tree;
 		init_tree_desc(&trees[1], &tree->object.oid,
 			       tree->buffer, tree->size);
@@ -913,7 +913,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 						     work,
 						     old_tree);
 			if (ret < 0)
-				exit(128);
+				die(NULL);
 			ret = reset_tree(new_tree,
 					 opts, 0,
 					 writeout_error, new_branch_info);

builtin/fetch.c

@@ -992,7 +992,7 @@ static int update_local_ref(struct ref *ref,
 		fast_forward = repo_in_merge_bases(the_repository, current,
 						   updated);
 		if (fast_forward < 0)
-			exit(128);
+			die(NULL);
 		forced_updates_ms += (getnanotime() - t_before) / 1000000;
 	} else {
 		fast_forward = 1;