ExternalProject: Fix git submodule recursion on update

In commit 5bc6230741 (ExternalProject: Option to turn off recursive
update of git submodules, 2019-10-16) we implemented the feature in the
clone script written by `_ep_write_gitclone_script` but not in the
update script written by `_ep_write_gitupdate_script`.  Implement the
latter by factoring out a common helper to use in both places.

Fixes: #20335

Modules/ExternalProject.cmake

@@ -1239,7 +1239,7 @@ endif()
 endfunction()
 
 
-function(_ep_write_gitupdate_script script_filename git_EXECUTABLE git_tag git_remote_name init_submodules git_submodules git_repository work_dir)
+function(_ep_write_gitupdate_script script_filename git_EXECUTABLE git_tag git_remote_name init_submodules git_submodules_recurse git_submodules git_repository work_dir)
   if("${git_tag}" STREQUAL "")
     message(FATAL_ERROR "Tag for git checkout should not be empty.")
   endif()
@@ -2339,6 +2339,29 @@ function(_ep_is_dir_empty dir empty_var)
   endif()
 endfunction()
 
+function(_ep_get_git_submodules_recurse git_submodules_recurse)
+  # Checks for GIT_SUBMODULES_RECURSE property
+  # Default is ON, which sets git_submodules_recurse output variable to "--recursive"
+  # Otherwise, the output variable is set to an empty value ""
+  get_property(git_submodules_recurse_set TARGET ${name} PROPERTY _EP_GIT_SUBMODULES_RECURSE SET)
+  if(NOT git_submodules_recurse_set)
+    set(recurseFlag "--recursive")
+  else()
+    get_property(git_submodules_recurse_value TARGET ${name} PROPERTY _EP_GIT_SUBMODULES_RECURSE)
+    if(git_submodules_recurse_value)
+      set(recurseFlag "--recursive")
+    else()
+      set(recurseFlag "")
+    endif()
+  endif()
+  set(${git_submodules_recurse} "${recurseFlag}" PARENT_SCOPE)
+
+  # The git submodule update '--recursive' flag requires git >= v1.6.5
+  if(recurseFlag AND GIT_VERSION_STRING VERSION_LESS 1.6.5)
+    message(FATAL_ERROR "error: git version 1.6.5 or later required for --recursive flag with 'git submodule ...': GIT_VERSION_STRING='${GIT_VERSION_STRING}'")
+  endif()
+endfunction()
+
 
 function(_ep_add_download_command name)
   ExternalProject_Get_Property(${name} source_dir stamp_dir download_dir tmp_dir)
@@ -2431,23 +2454,7 @@ function(_ep_add_download_command name)
       message(FATAL_ERROR "error: could not find git for clone of ${name}")
     endif()
 
-    get_property(git_submodules_recurse_set TARGET ${name} PROPERTY _EP_GIT_SUBMODULES_RECURSE SET)
-    if(NOT git_submodules_recurse_set)
-      set(git_submodules_recurse "--recursive")
-    else()
-      get_property(git_submodules_recurse_value TARGET ${name} PROPERTY _EP_GIT_SUBMODULES_RECURSE)
-      if(git_submodules_recurse_value)
-        set(git_submodules_recurse "--recursive")
-      else()
-        set(git_submodules_recurse "")
-      endif()
-    endif()
-
-    # The git submodule update '--recursive' flag requires git >= v1.6.5
-    #
-    if(git_submodules_recurse AND GIT_VERSION_STRING VERSION_LESS 1.6.5)
-      message(FATAL_ERROR "error: git version 1.6.5 or later required for 'git submodule update --recursive': GIT_VERSION_STRING='${GIT_VERSION_STRING}'")
-    endif()
+    _ep_get_git_submodules_recurse(git_submodules_recurse)
 
     get_property(git_tag TARGET ${name} PROPERTY _EP_GIT_TAG)
     if(NOT git_tag)
@@ -2774,8 +2781,10 @@ function(_ep_add_update_command name)
       endif()
     endif()
 
+    _ep_get_git_submodules_recurse(git_submodules_recurse)
+
     _ep_write_gitupdate_script(${tmp_dir}/${name}-gitupdate.cmake
-      ${GIT_EXECUTABLE} ${git_tag} ${git_remote_name} ${git_init_submodules} "${git_submodules}" ${git_repository} ${work_dir}
+      ${GIT_EXECUTABLE} ${git_tag} ${git_remote_name} ${git_init_submodules} "${git_submodules_recurse}" "${git_submodules}" ${git_repository} ${work_dir}
       )
     set(cmd ${CMAKE_COMMAND} -P ${tmp_dir}/${name}-gitupdate.cmake)
     set(always 1)