[#2359] Update master before merge.

Problem
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PQM does not update master before merge and so the merge local branch can not be pushed to origin.


Changes
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change code to update before merge


How to test
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reviewers: @alibotean 

I don't know how to test this as this would require to change PQM slaves... 
before merging this, I will go to pqm slaves and will brink them to an older version... and check that merge is done with latest.

Thanks