[#2330] Fix merge_init to not depend on branch name.

Problem
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The git repo from build slave is based on SHA and does not waste time with branch names

this makes is hard to do various administrative git operations which depend on branch names

Changes
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Update initial merge command to use SHA instead of branch name


How to test
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reviewer: @alibotean 

nothing special, just to let you know about the the changes.

Hope we will no longer need to merge with master before calling PQM

brink is already release, and used in server