Merge the r1520065 group from trunk:

 * r1520065, r1520745
   Resolve a user triggerable assertion when during an update a file external
   is being replaced by a server excluded (aka absent) node.  Issue 4422.
   Justification:
     Quite a few reports of this assertion on the users@s.a.o list since 1.7.0.
   Votes:
     +1: rhuijben, breser, philip


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1534705 13f79535-47bb-0310-9956-ffa450edef68