Skip validation for changes with merge conflicts

When a change is flagged by Gerrit as mergeable=false, there is no
value in starting a series of validation jobs because they would
eventually fail as well.

Change-Id: Ie63cefb43f333dcf588cbc70898541551ea5d93f
diff --git a/jenkins/gerrit-verifier-flow.groovy b/jenkins/gerrit-verifier-flow.groovy
index 35af137..7d82057 100644
--- a/jenkins/gerrit-verifier-flow.groovy
+++ b/jenkins/gerrit-verifier-flow.groovy
@@ -68,6 +68,10 @@
   return params.collect { it.getParameter("CHANGE_ID").value }
 }
 
+def changeUrl(change) {
+  "${Globals.gerrit}${change}"
+}
+
 def acceptedChanges = changesJson.findAll {
   change ->
   sha1 = change.current_revision
@@ -76,6 +80,11 @@
       return false
   }
 
+  if(!change.mergeable) {
+      println "[WARNING] Skipping change ${changeUrl(change._number)} because has merge conflicts"
+      return false
+  }
+
   if(change.hashtags.contains("skipci")) {
       println "Skipping SHA1 $sha1 because it is tagged with #skipci"
       return false