Part of this test does "git pull" on a dirty work tree. We need to make
sure that 'branch.master.rebase' is false for the test repository.
Otherwise if it is true in the user configuration then pull will refuse
to rebase and the test will fail.
This creates cmCTestGIT to drive CTest Update handling on git-based work
trees. Currently we always update to the head of the remote tracking
branch (git pull), so the nightly start time is ignored for Nightly
builds. A later change will address this. See issue #6994.