Alter branch name when notifying on PR commits

[monitorjbl]

The branch name system works great for single repos, but it kind of falls apart when forks are involved. Because the PR commit only uses the branch name from the source repo, a fork can trigger Jenkins jobs that should not be triggered. For example, if the fork uses the branch name master, any jobs that are meant to build merged code will run on the unmerged PR code.

It seems like it would be fairly trivial to fix this by prepending the forked repo name to the branchname, or even simply sticking a pr- in front of the name. That way, Jenkins jobs could be configured to only run on PRs.

[dodok1]

IMO, there is #172 covering this use case.