storage:vcs:hg: remove grandchildren from Hg._vcs_listdir.

storage:vcs:hg: remove grandchildren from Hg._vcs_listdir.

This fixes:

  From: Phil Schumm
  Date: Thu, 23 Aug 2012 08:15:21 -0500
  Subject: [Be-devel] -be diff- under Hg

  ...
  I've just noticed that -be diff- seems to yield different output
  under Git than it does under Hg (e.g., in a Mercurial +repository,
  -be diff- appears to show all changes as new bugs rather than
  modified).
  ...