Push the membuffer cache L1 retention rate even further up to
almost perfect short-term caching.  When we need to drop an entry
due to index table overflow, drop L2 entries rather than L1 ones.

* subversion/libsvn_subr/cache-membuffer.c
  (find_entry): don't drop L1 entries when there are also L2 ones

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1519278 13f79535-47bb-0310-9956-ffa450edef68