On the fsfs-fromat7 branch:  Allow for modifying and re-evaluating
the top element of a priority queue.  This function not only saves
copy operations but it can also be more efficient to update the
queue position for the previous head than doing a full insertion.

* subversion/include/svn_sorts.h
  (svn__priority_queue_update): declare new function

* subversion/libsvn_subr/sorts.c
  (svn__priority_queue_update): implement it


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1467382 13f79535-47bb-0310-9956-ffa450edef68