[#2069] Fix PQM merge_init and add test_review task.

Problem
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PQM fails with silly ```You can not merge the main branches.``` error


Why we got this
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Buildbot use a fast and slim version of repo and it does not updates branch names and other references.
This is why  the merge_init script can not get branch name from git repo.


Changes
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Update code to get branch name from env variable.

Also add a dedicated test_review command to be used for sending a test for review. This make sure proper builder is selected and not local patch is sent to test remote.

How to test
--------------

reviewers: @alibotean 

Check that changes look good.

If code is not ok, I should not be able to merge this.

Thakns!