On the 'move-tracking-2' branch: Refactoring.

* subversion/svnmover/svnmover.c
  (svn_branch_revision_root_t): New, inserted between 'repos' and 'family'
    in the hierarchy.
  (svn_branch_revision_root_create): New.
  (... everywhere else ...): Adjust.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1629216 13f79535-47bb-0310-9956-ffa450edef68