Teach fsfsfixer.py to understand sharded repositories -- not automatically,
just using a config variable in the code.

* contrib/server-side/fsfsfixer/fixer/find_good_id.py
  (REVS_PER_SHARD): New global.
  (rev_file_path): Support sharding.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1441446 13f79535-47bb-0310-9956-ffa450edef68