Fix a key mapping inefficiency in simplified scheme used by the
1.9 membuffer cache.  In cases of fixed length 16 byte key structures,
the first bytes usually contain a revision number.  Rotating that before
mapping it onto index buckets creates unnecessary "hot buckets",
i.e. local index overflows causing data eviction.

* subversion/libsvn_subr/cache-membuffer.c
  (combine_key): don't rotate the first 8-bytes of a 16 byte key

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1489850 13f79535-47bb-0310-9956-ffa450edef68