fetch PR refspec for docker jenkins builds

tools/jenkins/run_jenkins.sh

@@ -41,9 +41,17 @@ if [ "$platform" == "linux" ]
 then
   echo "building $language on Linux"
 
+  if [ "$ghprbPullId" != "" ]
+  then
+    # if we are building a pull request, grab corresponding refs.
+    FETCH_PULL_REQUEST_CMD="&& git fetch $GIT_URL +refs/pull/$ghprbPullId:refs/remotes/origin/pr/$ghprbPullId"
+  fi
+
   # Run tests inside docker
   docker run grpc/grpc_jenkins_slave bash -c -l "git clone --recursive $GIT_URL /var/local/git/grpc \
-    && cd /var/local/git/grpc && git checkout -f $GIT_COMMIT \
+    && cd /var/local/git/grpc \
+    $FETCH_PULL_REQUEST_CMD \
+    && git checkout -f $GIT_COMMIT \
     && git submodule update \
     && nvm use 0.12 \
     && rvm use ruby-2.1 \