log: Don't disable loggers when loading fileConfig

The default when loading config for logging from a file is to disable
existing loggers. Since some loggers are created before logging is set
up, these loggers were disabled if logging/config_file is set.

docs/changelog.rst

@@ -13,6 +13,10 @@ v0.20.0 (UNRELEASED)
 - Add cover URL to all scanned files with MusicBrainz album IDs. (Fixes:
   :issue:`697`, PR: :issue:`802`)
 
+**Logging**
+
+- Fix that some loggers would be disabled if logging/config_file was set.
+
 
 v0.19.4 (UNRELEASED)
 ====================

mopidy/utils/log.py

@@ -47,7 +47,8 @@ def setup_logging(config, verbosity_level, save_debug_log):
     if config['logging']['config_file']:
         # Logging config from file must be read before other handlers are
         # added. If not, the other handlers will have no effect.
-        logging.config.fileConfig(config['logging']['config_file'])
+        logging.config.fileConfig(config['logging']['config_file'],
+                                  disable_existing_loggers=False)
 
     setup_console_logging(config, verbosity_level)
     if save_debug_log: