Use unicode_output=False in some Darcs._u_invoke_client() calls.

Use unicode_output=False in some Darcs._u_invoke_client() calls.

This avoids the following error:
  ======================================================================
  ERROR: Should get file contents as committed to specified revision.
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File ".../libbe/vcs.py", line 860, in test_revision_file_contents_as_committed
      full_path, revision)
    File ".../libbe/vcs.py", line 339, in get_file_contents
      contents = self._vcs_get_file_contents(relpath,revision,binary=binary)
    File ".../libbe/darcs.py", line 122, in _vcs_get_file_contents
      status,output,error = self._u_invoke(args, stdin=major_patch)
    File ".../libbe/vcs.py", line 488, in _u_invoke
      raise CommandError(args, status, stdout, stderr)
  CommandError: Command failed (2):
    patch: **** Only garbage was found in the patch input.

  while executing
    ['patch', '--reverse', 'a/text']

After adding the unicode_output=False lines, I adjusted the
VCS._u_invoke_client() definition to pass all it's kwargs
automatically through to VCS._u_invoke().  To make this simpler and
more consistent, I renamed the "directory" option to "cwd", and
adjusted *._u_invoke() calls appropriately in several VCS backends.