Code review comment for lp://staging/~beuno/loggerhead/global-config

This branch allows setting configs in ~/.bazaar/bazaar.conf, which are overridden by the command line.
I think this branch has 3 things to consider:

1. Testing. I can't figure out how to test it, as all options seem to excersized by serve-branches, which isn't invoked in tests

2. Add a big fat deprecation warning to start-loggerhead script

3. I'm not 100% sure how to (properly) detect when something is set on the command line, so I hacked my way around it.

Any help is very much appreciated