If GIT_EXECUTABLE is not passed in, and is not available from
DartConfiguration.tcl or CTestConfiguration.ini, then make one
more last ditch attempt to get it from Update.xml, if there is
an Update.xml. For dashboards that have successfully done a
ctest_update call, there should be an Update.xml in the Testing
subdir of the binary tree. Parse that file for the git executable
recorded in the <UpdateCommand> element.

And make this test pass on those RogueResearch dashboard machines!