Use target branch in merge request lookup

The tests in #2081585: [META] Modernize the History module look like they are targeting MR !16 instead of !17.

DrupalorgIssueFork::getMergeRequestIID() looks up the merge request by source branch. In this case, both merge requests have the same source branch. We should also be checking the target branch, which is different between the two merge requests.

We don’t have the target branch in the DB right now, so we need to add a column to drupalorg_issue_fork_branches and back-fill that data.