On the 'move-tracking-2' branch: When reading branching metadata, share the
same branch sibling definition objects across all revisions.

* BRANCH-README
  Remove this from the to-do list.

* subversion/libsvn_delta/compat3.c
  Implement the change.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1639314 13f79535-47bb-0310-9956-ffa450edef68