If membuffer caches are being disabled, make sure that mandatory
caches are still being created (using the 1.6 in-process cache).

FSFS format 7 index cache code never skips the cache lookup and
update.  Allowing for NULL index caches would at least tripple
the amount of I/O (opening 3 files instead of one to access data).
So, those caches are mandatory and even small ones will help.

* subversion/libsvn_fs_fs/caching.c
  (svn_fs_fs__initialize_caches): Set in-process cache dimensions
                                  on index caches as well as other
                                  non-conditional caches - even if
                                  the using code would check for NULL.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1547134 13f79535-47bb-0310-9956-ffa450edef68