On the 'move-tracking-2' branch: Make 'svnmover diff' recurse into
subbranches.

* BRANCH-README
  Remove this to-do item.

* subversion/svnmover/svnmover.c
  (get_subbranches,
   svn_branch_diff_r): New.
  (execute): Use svn_branch_diff_r.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1639700 13f79535-47bb-0310-9956-ffa450edef68