Disable _REPOSITORY definitions.

These give the wrong result when the directory LLVM is being built in is inside
another Git repository (as in GARStow).

--- llvm-project-17.0.1.src/llvm/cmake/modules/VersionFromVCS.cmake	2023-09-19 10:05:13.000000000 +0100
+++ llvm-project-17.0.1.src/llvm/cmake/modules/VersionFromVCS.cmake	2023-09-19 10:36:47.967497793 +0100
@@ -5,7 +5,7 @@
 
 function(get_source_info path revision repository)
   find_package(Git QUIET)
-  if(GIT_FOUND)
+  if(GARSTOW_NOT_GIT_FOUND)
     execute_process(COMMAND ${GIT_EXECUTABLE} rev-parse --git-dir
       WORKING_DIRECTORY ${path}
       RESULT_VARIABLE git_result
--- llvm-project-17.0.1.src/llvm/utils/gn/build/write_vcsrevision.py	2023-09-19 10:05:13.000000000 +0100
+++ llvm-project-17.0.1.src/llvm/utils/gn/build/write_vcsrevision.py	2023-09-19 10:54:20.577928108 +0100
@@ -45,7 +45,7 @@
     args = parser.parse_args()
 
     vcsrevision_contents = ""
-    if args.write_git_rev:
+    if False and args.write_git_rev:
         git, use_shell = which("git"), False
         if not git:
             git = which("git.exe")