On the revprop-caching-ng branch:  Allow for read-only repositories.

Open the revprop generation file in r/o mode by default and require
write access only upon the first write attempt.

* subversion/libsvn_fs_fs/revprops.c
  (close_revprop_generation_file): Move up here.
  (ensure_revprop_generation): Add r/o option and re-open the file if
                               write is requested for the first time.
  (read_revprop_generation_file,
   write_revprop_generation_file, 
   has_revprop_cache): Update callers.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/revprop-caching-ng@1620240 13f79535-47bb-0310-9956-ffa450edef68