Fix ClearCase unextend_path() on Windows.

The ntpath in Python behaves in the following manner:
  >>> ntpath.join('c:', 'dir', 'file')
  'c:dir\\file'

  >>> os.chdir('C:\\Windows')
  >>> ntpath.realpath(ntpath.join('C:', 'dir', 'file'))
  'C:\\Windows\\dir\\file'

  >>> os.chdir('C:\\Windows')
  >>> ntpath.realpath(ntpath.join('C:\\', 'dir', 'file'))
  'C:\\dir\\file'

  >>> ntpath.realpath(ntpath.join('/', 'computer', 'share', 'file'))
  'C:\\computer\\share\file'

  >>> ntpath.realpath(ntpath.join('\\\\', 'computer', 'share', 'file'))
  '\\\\computer\\share\\file'

The unextended_chunks[0] in case of repository on local drive is 'X:' where X is the drive letter, and in case of UNC path is '/'.
Change the unextended_chunks[0] so the returned realpath is correct.

This both makes the displayed path proper and resolves the "ValueError: Cannot mix UNC and non-UNC paths" when Clear Case repository is configured with UNC path.

Tested on production 1.6.11 installation.