Correct for possible directory changes in mercurial.dispatch.dispatch()

Correct for possible directory changes in mercurial.dispatch.dispatch()

I ran across this when the hg unittests broke the vcs.base unittests:
  $ python test.py libbe.storage.vcs.base libbe.storage.vcs.hg
  ...
  OK
  $ python test.py libbe.storage.vcs.hg libbe.storage.vcs.base
  ...
  File ".../libbe/storage/vcs/base.py", line 914, in libbe.storage.vcs.base.VCSTestCase.Class._u_rel_path
  Failed example:
      vcs._u_rel_path("./a", ".")
  Exception raised:
      Traceback (most recent call last):
        File "/usr/lib/python2.5/doctest.py", line 1228, in __run
          compileflags, 1) in test.globs
        File "<doctest libbe.storage.vcs.base.VCSTestCase.Class._u_rel_path[4]>", line 1, in <module>
          vcs._u_rel_path("./a", ".")
        File ".../libbe/storage/vcs/base.py", line 921, in _u_rel_path
          path = os.path.abspath(path)
        File "/usr/lib/python2.5/posixpath.py", line 403, in abspath
          path = join(os.getcwd(), path)
      OSError: [Errno 2] No such file or directory
  ...
  FAILED (failures=1)