Merge r1541635 from trunk:

 * r1541635
   Allow using 'svn diff F:/' for those poor Windows users that have a working
   copy in the root of the drive.
   Justification:
     Endless recursion is bad, especially in case some users might get into
     this in a pretty normal code path.
   Votes:
     +1: rhuijben, philip, stsp


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1543706 13f79535-47bb-0310-9956-ffa450edef68