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

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

* subversion/include/private/svn_editor3.h
  (svn_branch_repos_t): Add a global list of branch family objects here.
  (svn_branch_revision_root_t): Move the list of branch instance objects to
    here...
  (svn_branch_family_t): ... from here.
  (svn_branch_family_get_children): New.

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

* subversion/libsvn_ra/ra_loader.c
  (default_repos_info): Tweak the initial metadata.

* subversion/svnmover/svnmover.c
  (family_list_branch_instances,
   execute): Update accordingly.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1639306 13f79535-47bb-0310-9956-ffa450edef68