Logging utilities for Dulwich.
Any module that uses logging needs to do compile-time initialization to set up the logging environment. Since Dulwich is also used as a library, clients may not want to see any logging output. In that case, we need to use a special handler to suppress spurious warnings like “No handlers could be found for logger dulwich.foo”.
For details on the _NullHandler approach, see: http://docs.python.org/library/logging.html#configuring-logging-for-a-library
For many modules, the only function from the logging module they need is getLogger; this module exports that function for convenience. If a calling module needs something else, it can import the standard logging module directly.
Set up the default Dulwich loggers.
Remove the null handler from the Dulwich loggers.
If a caller wants to set up logging using something other than default_logging_config, calling this function first is a minor optimization to avoid the overhead of using the _NullHandler.