On the 'move-tracking-2' branch: A follow-up fix and tweak to r1639700.

* subversion/svnmover/svnmover.c
  (svn_branch_diff_r): Fix a crash in the 'branch deleted' case. Tweak the
    displayed messages.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1639710 13f79535-47bb-0310-9956-ffa450edef68