Tests: Fix RunCMake.ctest_update test to not require GIT_EXECUTABLE

Fix the test case added by commit 95db11fdb1 (ctest: Include ChangeId in
Update.xml, 2019-04-23) to work in a release branch of CMake.
The `GIT_EXECUTABLE` value is not set before we add this test in
release branch builds.

Tests/RunCMake/CMakeLists.txt

@@ -259,7 +259,7 @@ add_RunCMake_test(ctest_submit)
 add_RunCMake_test(ctest_test)
 add_RunCMake_test(ctest_disabled_test)
 add_RunCMake_test(ctest_skipped_test)
-add_RunCMake_test(ctest_update -DGIT_EXECUTABLE=${GIT_EXECUTABLE})
+add_RunCMake_test(ctest_update)
 add_RunCMake_test(ctest_upload)
 add_RunCMake_test(ctest_fixtures)
 add_RunCMake_test(file)

Tests/RunCMake/ctest_update/test.cmake.in

@@ -9,7 +9,9 @@ set(CTEST_CMAKE_GENERATOR               "@RunCMake_GENERATOR@")
 set(CTEST_CMAKE_GENERATOR_PLATFORM      "@RunCMake_GENERATOR_PLATFORM@")
 set(CTEST_CMAKE_GENERATOR_TOOLSET       "@RunCMake_GENERATOR_TOOLSET@")
 set(CTEST_BUILD_CONFIGURATION           "$ENV{CMAKE_CONFIG_TYPE}")
-set(CTEST_UPDATE_COMMAND                "@GIT_EXECUTABLE@")
+
+# FIXME: update test to do someting meaningful with this.
+set(CTEST_UPDATE_COMMAND                "not-actually-used")
 
 set(ctest_test_args "@CASE_CTEST_UPDATE_ARGS@")
 ctest_start(Experimental)