Create backport branch for r1625533 group

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-r1625533@1640743 13f79535-47bb-0310-9956-ffa450edef68
Merge r1625533,r1625537 from trunk, use svn__apr_hash_index_key.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-r1625533@1640744 13f79535-47bb-0310-9956-ffa450edef68
Merge the 1.8.x-r1625533 branch:

 * r1625533, r1625537
   Keep externals registrations on 'svn upgrade' (part of issue 4519)
   Justification:
     'svn upgrade' shouldn't destroy valid information.
   Branch:
     ^/subversion/branches/1.8.x-r1625533
   Votes:
     +1: rhuijben, philip, stefan2


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1642633 13f79535-47bb-0310-9956-ffa450edef68