Git does not automatically checkout the matching version of a submodule
when it checks out a new version of the parent project in the work tree.
If the submodule reference changed in the parent project then we were
reporting the submodule path as a local modification.  Work around the
problem in ctest_update using "git submodule update" after "git pull".
For projects with no submodules this is a no-op.  See issue #10662.
Also add a submodule to the test project for CTest.UpdateGIT to test the
work-around.