Add 'config' parameter to Git source build step to enable
git-config settings when running the low-level git commands.
Merge branch 'git_config' of git://github.com/andrewjcg/buildbot
Pass tryclient properties to getAvailableBuilderNames call
Merge branch 'try_client_available_builder_names' of git://github.com/andrewjcg/buildbot

(with pyflakes fix)