On the 'move-tracking-2' branch: A small bug fix.

* subversion/libsvn_delta/branching.c
  (svn_branch_map_branch_children): As the doc string says, FROM_BRANCH and
    TO_BRANCH must be different branch instances, which in nested branching
    does not necessarily imply different sibling-ids. Correct the assertion.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1640130 13f79535-47bb-0310-9956-ffa450edef68