propose_update.sh: Don't output complete git diff

When this script is invoked from the propose-openstack-manuals-updates
job, it can easily generate several 10k lines of diff.

The git diff command outputs all of them.

Use "git diff --stat" instead for a very brief output.

Change-Id: I065fefbf68b009931899065ab65b2f8167f9e171

modules/openstack_project/files/slave_scripts/propose_update.sh

@@ -97,7 +97,7 @@ EOF
         fi
 
         pushd $PROJECT_DIR
-        if ! git diff --exit-code HEAD ; then
+        if ! git diff --stat --exit-code HEAD ; then
             # Commit and review
             git_args="-a -F-"
             git commit $git_args <<EOF