Silence git-hooks/pre-commit in the absence of .gitmodules (take 2).

build/git-hooks/pre-commit

 #!/bin/sh
 
 submodule_diff() {
   if test -n "$2"; then
     git diff-tree -r --ignore-submodules=dirty "$1" "$2" | grep -e '^:160000' -e '^:...... 160000' | xargs
   else
     git diff-index --cached --ignore-submodules=dirty "$1" | grep -e '^:160000' -e '^:...... 160000' | xargs
   fi
 }
 
@@ skipping 31 matching lines @@
 If you're really and truly trying to roll the version of a submodule, you should
 commit the new version to DEPS, instead.
 EOF
   exit 1
 fi
 
 gitmodules_diff() {
   git diff-index --cached "$1" .gitmodules
 }
 
-if test "$(git ls-files .gitmodules)" -a "$(gitmodules_diff $head_ref)"; then
+if [ "$(git ls-files .gitmodules)" ] && [ "$(gitmodules_diff $head_ref)" ]; then
   cat <<EOF 1>&2
 You are trying to commit a change to .gitmodules.  That is not allowed.
 To make changes to submodule names/paths, edit DEPS.
 EOF
   exit 1
 fi
 
 exit 0