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

Other projects use depot_tools (which pulls in this hook) and chromium's build/
(where the hook lives) but don't use git modules (e.g. webrtc).

TBR=szager@chromium.org

Review URL: https://codereview.chromium.org/196413021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257470 0039d316-1c4b-4281-b951-d872f2087c98

build/git-hooks/pre-commit

@@ -23,7 +23,8 @@ 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.