ENH: Teach CTest to handle Mercurial repositories
This creates cmCTestHG to drive CTest Update handling on hg-based work trees. Currently we always update to the head of the remote tracking branch (hg pull), so the nightly start time is ignored for Nightly builds. A later change will address this. See issue #7879. Patch from Emmanuel Christophe. I modified the patch slightly for code style, to finish up some parsing details, and to fix the test.