Merge the r1537360 group from trunk:

 * r1537360, r1537415, r1537555, r1537700
   mod_dav_svn: Set r->filename to a bogus value rather than NULL.
   Justification:
     Some httpd modules (mod_wsgi) assume that r->filename is not NULL and
     don't bother to check before dereferincing.  Regression from earlier
     1.8.x releases and user found server crash.
   Votes:
     +1: breser, rhuijben, brane


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1538141 13f79535-47bb-0310-9956-ffa450edef68
Merge r1525902 from trunk:

 * r1525902
   Fix issues #4428, #4429: "checkout via http with external pointing at
   redirected URL".
   Justification:
     Such a stupid mistake, to carefully follow the URL redirection just to
     use the unredirected URL subsequently. Breaks externals as soon as the
     repository URL gets redirected.
   Notes:
     I'd be glad to add a regression test, but I'm not entirely clear on how
     to make our test suite model an external to a redirected URL. A testing
     recipe is in issue #4428.
   Votes:
     +1: neels, rhuijben, brane


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1538142 13f79535-47bb-0310-9956-ffa450edef68