Membuffer cache supports NULL keys.  Make the debugging code support
them as well instead of segfaulting.

* subversion/libsvn_subr/cache-membuffer.c
  (DEBUG_CACHE_MEMBUFFER_INIT_TAG): handle NULL keys properly

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1520423 13f79535-47bb-0310-9956-ffa450edef68
Address a subtle performance issue with the block read feature in FSX
and the upcoming FSFS format 7.

If some data cannot be found in the cache, we read a whole block from
the pack / rev file or some index.  If we already read that block in
the past, most items will still be cached and we won't parse them again.
However, that sets those old entries at a disadvantage since they have
often not been hit - yet - and may get evicted from L2 soon.

By counting a "has key" access as a hit, we give existing entries from
the respective block some added protection - although not as high as
the missing ones will get from being put into L1.

* subversion/libsvn_subr/cache-membuffer.c
  (membuffer_cache_has_key_internal,
   membuffer_cache_has_key): count as read and record hits

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1520428 13f79535-47bb-0310-9956-ffa450edef68