[#1749] Merge PQM with squash.

Problem description
===================

With this new version PQM will merge branches with squash and will close the pull request.

Note that this works only for branches landed with PQM... it does not work for branches that are manually merged with squash.


Changes description
===================

when PQM commits the merge, it will also delete the remote original branch and this will trigger the pull request close action.

Added some brutal tests for manually closing the pull request, but the code is no longer required.
I have left is there since maybe we will use it in the future.
The tests for GitHub interaction require github authentication so I can not put the private credentials in the test.

I have tested the PQM and it worked.
Merged with squash and old branch removed.... and pull request closed.


How to try and test the changes
===============================

reviewers @alibotean 

This is just to let you know about the changes.

After this branch is landed on master and we start using brink 13.0 all merges will be with squash.