Merge the 1.8.x-issue4658 branch:

 * r1766352, r1766590
   Fix issue #4658: Can't reconstruct fulltext from DELTA against PLAIN in FSFS
   Justification:
     Prevents access to committed data.  This is as close to a corruption one
     could get without actually corrupting anything.  User reported issue.
   Branch:
     ^/subversion/branches/1.8.x-issue4658
   Notes:
     r1766352 contains the actual fix.
     r1766590 adds a test case for it.
   Votes:
     +1: stefan2, stsp, brane
     +0: rhuijben


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1770613 13f79535-47bb-0310-9956-ffa450edef68