Add missing break-lock action command-line help (#357).

borgmatic/commands/arguments.py

@@ -787,6 +787,7 @@ def make_parsers():
         '--repository',
         help='Path of repository to break the lock for, defaults to the configured repository if there is only one',
     )
+    break_lock_group.add_argument('-h', '--help', action='help', help='Show this help message and exit')
 
     borg_parser = subparsers.add_parser(
         'borg',