Do not use Jenkins pipeline default checkout

Jenkins pipeline default checkout has issues with
merge commits and also do not take into consideration
the Gerrit target branch where the patch-set is going
to be merge into.

Change-Id: I3633cb7da0b0461e74275a2572106e42b05e6509

Jenkinsfile

diff --git a/Jenkinsfile b/Jenkinsfile
index 41b0880..8b696b9 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -1,7 +1,13 @@
 pipeline {
-    options { checkoutToSubdirectory('zookeeper-refdb') }
+    options { skipDefaultCheckout true }
     agent { label 'bazel-debian' }
     stages {
+        stage('Checkout') {
+            steps {
+                sh "git clone -b ${env.GERRIT_BRANCH} https://gerrit.googlesource.com/plugins/zookeeper-refdb"
+                sh "cd zookeeper-refdb && git fetch origin refs/changes/${BRANCH_NAME} && git merge FETCH_HEAD"
+            }
+        }
         stage('Formatting') {
             steps {
                 sh "find zookeeper-refdb -name '*.java' | xargs /home/jenkins/format/google-java-format-1.7 -i"