Check that the target branch exists

* propose_requirements_update.sh: Look before we leap. Don't descend
into the project repository and try to check out a branch without
first making sure it exists on the origin.

Change-Id: Ib4de5bdf8bf5cf134e3307755617456f282d790d

modules/jenkins/files/slave_scripts/propose_requirements_update.sh

@@ -56,8 +56,10 @@ EOF
     PROJECT_DIR=$(basename $PROJECT)
     rm -rf $PROJECT_DIR
     git clone ssh://$USERNAME@review.openstack.org:29418/$PROJECT.git
-
     pushd $PROJECT_DIR
+
+    # make sure the project even has this branch
+    if git branch -a | grep -q "^  remotes/origin/$BRANCH$" ; then
         git checkout $BRANCH
         git review -s
         if [ -n "$change_id" ] ; then
@@ -79,6 +81,7 @@ $COMMIT_MSG
 EOF
             git review -t $TOPIC $BRANCH
         fi
-    popd
+    fi
 
+    popd
 done