COMP: Update CTestCustom to exclude "HEAD is now" git warning.

Despite the fact the git client on the different Slicer dashboard
client has been setup with "advice.detachedHead" warning disabled [1]

The "HEAD is now ..." output still show up as a warning on CDash.
See http://slicer.cdash.org/viewBuildError.php?type=1&buildid=103147

This commit attempts to suppress it.

[1] http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Tutorials/DashboardSetup#Setup

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22048 3bd1e089-480b-0410-8dfb-8563597acbee