Silence git-hooks/pre-commit in the absence of .gitmodules.

build/git-hooks/pre-commit

 #!/bin/sh
 
 submods=$(git diff-index --cached --ignore-submodules=dirty HEAD | grep -e '^:160000' -e '^:...... 160000' | xargs)
 if test "$submods"; then
   echo "You are trying to commit changes to the following submodules:" 1>&2
   echo 1>&2
   echo $submods | cut -d ' ' -f 6 | sed 's/^/  /g' 1>&2
   cat <<EOF 1>&2
 
 Submodule commits are not allowed.  Please run:
 
   git status --ignore-submodules=dirty
 
 and/or:
 
   git diff-index --cached --ignore-submodules=dirty HEAD
 
 ... to see what's in your index.
 
 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
 
-if test "$(git diff-index --cached HEAD .gitmodules)"; then
+if [[ -n "$(git ls-files .gitmodules)" &&
+      -n "$(git diff-index --cached HEAD .gitmodules)" ]]; 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