Follow-up to r1483434:  Tweak our hash function to increase the
likelihood of higher bits in a 32 bit key chunk to have any
impact on the result.

Found by: rhuijben

* subversion/libsvn_subr/hash.c
  (hashfunc_compatible): cheaply fold the current chunk before
                         feeding it into the; update comment 

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483471 13f79535-47bb-0310-9956-ffa450edef68