On the 'move-tracking-2' branch: Add a 'diff' command to 'svnmover'.

* subversion/svnmover/svnmover.c
  (svn_branch_diff): New.
  (execute): Call it.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1634891 13f79535-47bb-0310-9956-ffa450edef68