Prevent update_spirv_deps.sh from updating re2 and effcee. (#5250)

* Prevent update_spirv_deps.sh from updating re2 and effcee.

As per discussion in
https://github.com/microsoft/DirectXShaderCompiler/pull/5246, the
repositories re2 and effcee should not be updated when rolling
submodules forward.

* Also ignore DirectX-Headers submodule updates.

* Fix comment to match code.

utils/update_spirv_deps.sh

@@ -4,7 +4,16 @@ if [ ! -d external -o ! -d .git -o ! -d azure-pipelines ] ; then
   exit 1
 fi
 
-set -ex
-git submodule foreach 'set -x && git switch main && git pull --ff-only'
+# Ignore effcee, re2 and DirectX-Headers updates.  See the discussion at
+# https://github.com/microsoft/DirectXShaderCompiler/pull/5246
+# for details.
+git submodule foreach '                                                   \
+  n=$(basename $sm_path);                                                 \
+  if [ "$n" != "re2" -a "$n" != "effcee" -a "$n" != "DirectX-Headers" ];  \
+  then git switch main && git pull --ff-only;                             \
+  else echo "Skipping submodule $n";                                      \
+  fi;                                                                     \
+  echo                                                                    \
+  '
 git add external
 exit 0