Clobber if workdir exists but isn't a git checkout. Fixes #2531
Merge branch 'bug2531-sqash' of git://github.com/srinupiits/buildbot

Conflicts:
	master/buildbot/steps/source/git.py
	master/buildbot/test/unit/test_steps_source_git.py
	slave/buildslave/commands/base.py
	slave/buildslave/commands/registry.py
plus autopep8