Be explicit and verbose when using git diff.

Specify that git should diff against HEAD when diffing in the
propose_requirements_update.sh script. Replace --quiet with --exit-code
so that we can verbosely see what differences there are.

Change-Id: I0d194151db457f7eb56e010c94b20562d0221e34

modules/jenkins/files/slave_scripts/propose_requirements_update.sh

@@ -67,7 +67,7 @@ EOF
     python update.py $PROJECT_DIR
 
     pushd $PROJECT_DIR
-    if ! git diff --quiet ; then
+    if ! git diff --exit-code HEAD ; then
         # Commit and review
         git_args="-a -F-"
         if [ -n "$change_id" ] ; then