In FSX, use a faster method to verify that a given item is actually a
representation: Simply ask the index.

Because the indexes themselves get cross-checked against one another
and against the rev / pack contents, we have high confidence that this
is nothing but a representation (no overlaps, nothing missing, no
external corruption).  The contents, however, may still be invalid or
its checksum(s) may not match the ones in REP.  But we only need to
guarantee accessibility here.

As a result, we also don't need file HINTs anymore and can simplify
the verfication walker.

* subversion/libsvn_fs_x/cached_data.h
  (svn_fs_x__check_rep): No HINTs needed anymore.

* subversion/libsvn_fs_x/cached_data.c
  (svn_fs_x__check_rep): Simply check that REP can actually be found
                         as a representation in our indexes.

* subversion/libsvn_fs_x/transaction.c
  (get_shared_rep): Update caller to simplified API.

* subversion/libsvn_fs_x/verify.c
  (verify_walker_baton_t): Remove revision file hinting elements.
  (verify_walker,
   verify_rep_cache): Update / simplify users.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1598283 13f79535-47bb-0310-9956-ffa450edef68