Merge branch 'jc/maint-co-track'

* jc/maint-co-track:
  Enhance hold_lock_file_for_{update,append}() API
  demonstrate breakage of detached checkout with symbolic link HEAD
  Fix "checkout --track -b newbranch" on detached HEAD

Conflicts:
	builtin-commit.c

builtin-commit.c

@@ -320,7 +320,9 @@ static char *prepare_index(int argc, const char **argv, const char *prefix)
 		die("unable to write new_index file");
 
 	fd = hold_lock_file_for_update(&false_lock,
-				       git_path("next-index-%"PRIuMAX, (uintmax_t) getpid()), 1);
+				       git_path("next-index-%"PRIuMAX,
+						(uintmax_t) getpid()),
+				       LOCK_DIE_ON_ERROR);
 
 	create_base_index();
 	add_remove_files(&partial);