Improve usability of the fsfs-stats tool.

* tools/server-side/fsfs-stats.c
  (print_usage): cache size is now optional
  (main): default cache size if now given; sanitize repo path before using it

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1431263 13f79535-47bb-0310-9956-ffa450edef68