[JENKINS-20392] Fix "Recent History" for merge jobs.

To compute the recent-history, jenkins uses a hashmap of branch-names
and commit-ids. The bug has two sources:
1) To fill the hashmap, jenkins uses only the branch-name and
commit-ids of those commits, that triggered the build. The
merge-commits created by jenkins are ignored (see
BuildData.java#L129).
2) To calculate the "Recent Changes", jenkins searches for the
branch-names of the commit, that was built. But jenkins considers the
merge-commit not part of any branch (the call to getRevisionsForSHA1
in PreBuildMerge.java#L93 returns a Revision without associated
branches).

The patch fixes this bug by 1) including the branches of the current
commit in the recent-changes-hashmap and 2) associating the current
commit with the correct branch-name.