wrapper: clean the source dir before a checkout

Git doesn't clear folders containing untracked files when switching to
another branch. The autohelp script uses invalid .pyc files when such
switches happen in the diff_branches.py script. This change removes all
the .pyc files before a checkout to avoid this problem.

Change-Id: I0acd0308a3651b27d158546036de5cb0b42d974c