ExternalProject: omit --no-checkout from git clone when using git 2.20.x.

The optimization from commit 627fc5b44f (ExternalProject: Avoid
unnecessary checkout on clone, 2019-07-29, v3.16.0-rc1~325^2) triggers a
bug in the Git 2.20.x series that is not in older or newer versions.
Drop the optimization for that specific range of Git versions.

Fixes: #21009

Modules/ExternalProject.cmake

@@ -1124,7 +1124,11 @@ function(_ep_write_gitclone_script script_filename source_dir git_EXECUTABLE git
     message(FATAL_ERROR "Tag for git checkout should not be empty.")
   endif()
 
-  set(git_clone_options "--no-checkout")
+  if(GIT_VERSION_STRING VERSION_LESS 2.20 OR 2.21 VERSION_LESS_EQUAL GIT_VERSION_STRING)
+    set(git_clone_options "--no-checkout")
+  else()
+    set(git_clone_options)
+  endif()
   if(git_shallow)
     if(NOT GIT_VERSION_STRING VERSION_LESS 1.7.10)
       list(APPEND git_clone_options "--depth 1 --no-single-branch")