Add 'config' parameter to Git source build step to enable
git-config settings when running the low-level git commands.
tryclient: support commas in property CLI

This adds a '--property' flag which contains single a property setting
and can be set multiple times (allowing for comments in property values).
Merge branch 'try_properties' of git://github.com/andrewjcg/buildbot
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)
don't print error message until after checking exception type
Bugfix for ticket #2370, ShellCommand now checks validity of args during config rather than build
removed unused variables from unit test
Merge branch 'ticket2370' of git://github.com/ffledgling/buildbot