Change script behavior to always check linked bugs

[Ursula Junque]

Now the script checks for bugs in the following order:
1) PQM commit message;
2) Merged branch nick;
3) In the merged branch in Launchpad, look for linked bugs.

The desirable behavior is:
1) PQM commit message and linked bugs in Launchpad branches;
2) Merged branch nick.

This way we can start working with linked bugs more often, not just the commit message, to find out the fixed bugs.