GerritTrigger.equals/hashCode -- don't use AbstractProject

AbstractProjects are very temporary and shouldn't be used for comparison.
Instead, AbstractProject.getFullName() should be used.

JENKINS-10709: multiple builds are triggered for one change in Gerrit

This is an attempt to do *something* for bug
[JENKINS-10709](https://issues.jenkins-ci.org/browse/JENKINS-10709).
It's based on the comments of @jglick on [jenkinsci-dev mailing list](https://groups.google.com/forum/#!topic/jenkinsci-dev/CwbsCJAzcOY).

I'm not really a Java programmer, so I didn't fully remove the `myProject` variable (which
stores an `AbstractProject`).  I also didn't know how to fetch an `AbstractProject` from
Jenkins given only a `fullName`.