Prevent large representations from thrashing the caches in FSFS.

While the fulltext caches won't accept these representations because
the individual objects are too long, the individual deltas are cacheable
(assuming delta caching has been enabled).  Although deltas have very
low caching priority, they would eventually evict all other contents.

* subversion/libsvn_fs_fs/cached_data.c
  (create_rep_state_body): Estimate the size of delta data that would go
                           into caches. Don't use the caches if too large.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590380 13f79535-47bb-0310-9956-ffa450edef68