Clean up the disk when cloning fails

Clean up the disk when cloning fails

CloneCommand.call() has three stages: preparation, then the actual
clone (init/fetch), and finally maybe checking out the working
directory.

Restructure such that if we fail or are cancelled during the actual
clone (middle phase), we do clean up the disk again. This prevents
leaving behind a partial clone in an inconsistent state: either we
have a fully successfully built clone, or nothing at all.

Bug: 516303
Change-Id: I9b18c60f8f99816d42a3deb7d4a33a9f22eeb709
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>