Implement Arch._vcs_path()

Implement Arch._vcs_path()

Fixes VersionedStorage_commit_TestCase.test_get_previous_children.
Should have fixed
  VersionedStorage_commit_TestCase.test_get_previous_version
too, but 'tla file-find' is buggy:
  https://bugs.launchpad.net/ubuntu/+source/tla/+bug/513472

Also:
  * sort children in test_get_previous_children, since we shouldn't
    require a particular child order
  * unescape filenames in Arch._diff()
  * remove debugging prints from Arch._parse_diff()
  * remove silly blank line in git.py I'd stumbled across ;).