Fix Issue #4320.  File hinting breaks when reading from a transaction commiting
the revision equal to the repo shard size.

* subversion/libsvn_fs_fs/fs_fs.c
  (create_rep_state_body): Test that rev_hint is a valid revnum.

* subversion/tests/libsvn_fs_fs/fs-pack-test.c
  (file_hint_at_shard_boundry, test_funcs): Add regression test for Issue #4320.

Found by: danielsh



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1446467 13f79535-47bb-0310-9956-ffa450edef68