Begin fixing the 'svnfsfs stats' scalability issue with large f6 revisions.
Instead of reading the whole revision into a single buffer, we need to read
the items individually.

This patch makes the rev / pack file available as part of the revision_info_t
and uses it to read the representation headers from it.  Later patchs will
iteratively replace the other places that access to the rev content buffer.

* subversion/libsvn_fs_fs/stats.c
  (rep_stats_t): Offsets are now proper file offsets.
  (revision_info_t): Add the REV_FILE as a member.

  (open_rev_or_pack_file): Remove; the REV_FILE will be available instead.
  (get_file_size): Take a revision file instead of a plain APR file.
  (get_content): The file is always open now.
  (compare_representation_offsets,
   find_representation): The representation OFFSETs are now apr_off_t.
  (parse_representation): Same.  Also, remove the FILE_CONTENT access and
                          read from the rev / pack file instead.
  (read_noderev): Update caller.

  (read_phys_pack_file,
   read_phys_revision_file): Open, provide and close the REV_FILE member. 

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639319 13f79535-47bb-0310-9956-ffa450edef68