Add locking of borgmatic config file

This prevents multiple instances of borgmatic from running against
the same config file. This is particularly important when there are
pre-backup scripts with side effects

borgmatic/commands/borgmatic.py

@@ -566,9 +566,25 @@ def main():  # pragma: no cover
     logger.debug('Ensuring legacy configuration is upgraded')
     convert.guard_configuration_upgraded(LEGACY_CONFIG_PATH, config_filenames)
 
+    locks = []
+    for config_filename,config in configs.items():
+        lock_config = config.get("location",dict()).get("lock_config",False)
+        if lock_config:
+            f = open(config_filename)
+            try:
+                fcntl.flock(f.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+            except IOError:
+                logger.critical("Failed to acquire lock for {}".format(config_filename))
+                sys.exit(0))
+            locks.append(f)
+
     summary_logs = parse_logs + list(collect_configuration_run_summary_logs(configs, arguments))
     summary_logs_max_level = max(log.levelno for log in summary_logs)
 
+    # this removes the reference to the open files which python will garbage collect the file objects
+    # and close them
+    locks = None
+
     for message in ('', 'summary:'):
         log_record(
             levelno=summary_logs_max_level,