feature finish: Merge branch and then update PT

    The order of actions taken was to

      1) Finish story in Pivotal Tracker.
      2) Merge the branches.

    If, however, the feature branch could not be merged, state of the
    story was updated in Pivotal Tracker anyway.

    This patch makes sure that the branch is merged before the state is
    updated in Pivotal Tracker.

Change-Id: I816eb171f986dfa2f9e252b5bfcecdafdae2e798
Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>