On the 'move-tracking-2' branch: Add some checking.

* subversion/svnmover/svnmover.c
  (svn_branch_mkdir): Check the target is in the specified branch.
  (svn_branch_mv): Add a to-do comment.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1627527 13f79535-47bb-0310-9956-ffa450edef68