[#2030] Fix PQM merge init.

Problem description
===================

PQM checks if it can merge the branch but then fails to revert the state.




Changes description
===================

The problem was caused after we switched to squash...
in the old code I have used 'merge --abort' to revert the merge state, but with the new '--squash' command, got does not do a proper merge , so 'merge --abort' no longer works since there is no merge

As a quick fix to have PQM working I have removed the 'merge --abort' command and things looked ok at first... until we had commits after merge.

I am now using 'git reset --hard' to reset the `to-be-merged` changes.


How to try and test the changes
===============================

reviewers @alibotean 

On this branch run:

```
./paver.sh merge_init GITHUB_TOKEN 31
```
You should see that it switches to master and then back to original branch.

I can not put token here since this is public and token should be secret.
You can get token from buildbot  PQM merge_init stdio.

Feel free to not test this code and just note the changes and trust me on this.

I placed the how to test info to have them for the records

Also, if the new code does not work, it would not be possible to merge this branch as it contains a ne commit after merging with master.

Thanks!