Use amend when updating reqs proposal change.

Use git commit --amend when updating an preexisting requirements
proposal update change. Otherwise multiple changes are proposed causing
git review to run interactively and fail (and if it didn't fail multiple
dependent changes would be proposed which we don't want either).

Change-Id: Ic5a173d671ebe25e2bcf2b4972e39879992d3bdf

modules/jenkins/files/slave_scripts/propose_requirements_update.sh

@@ -69,7 +69,11 @@ EOF
     pushd $PROJECT_DIR
     if ! git diff --quiet ; then
         # Commit and review
-        git commit -a -F- <<EOF
+        git_args="-a -F-"
+        if [ -n "$change_id" ] ; then
+            git_args="--amend $git_args"
+        fi
+        git commit $git_args <<EOF
 $COMMIT_MSG
 EOF
         git review -t $TOPIC $BRANCH