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.
Brad King