Merge the r1550691 group from trunk:

 * r1550691, r1550772, r1600909
   Guard against MD5 hash collisions when finding cached credentials.
   Justification:
     MD5 collision attacks exist and could be used to trick a client into
     sending cached credentials to a server other than what they were
     cached for.
   Votes:
     +1: breser, danielsh, rhuijben


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1605944 13f79535-47bb-0310-9956-ffa450edef68