When testing pull requests do a rebase to the merge point

These instructions can go in the pull job config rather than narayana.sh I think as they do a full clean and that would be dangerous on a dev box

1. Clean up the local repo
   git rebase --abort
   rm -rf .git/rebase-apply
   git clean -f -d -x

1. Work out the branch point
   git branch -D 4.17
   git branch 4.17 origin/4.17
   git branch -D master
   git branch master origin/master
   myRev=`git rev-parse HEAD`
   ancestor417=`git merge-base $myRev 4.17`
   ancestorMaster=`git merge-base $myRev master`
   distanceFromMaster=`git log $ancestorMaster..$myRev | grep commit | wc | cut -c 1-7 | tr -d ' '`
   distanceFrom417=`git log $ancestor417..$myRev | grep commit | wc | cut -c 1-7 | tr -d ' '`
   if [ "$distanceFromMaster" -lt "$distanceFrom417" ]
   then
   export BRANCHPOINT=master
   else
   export BRANCHPOINT=4.17
   fi

1. Update the pull to head
   git pull --rebase --ff-only origin $BRANCHPOINT
2. if this fails ($? -ne 0) fail the build and tell the committer (commentOnPull) that they need to rebase