Plug a hole in the svn_fs_fs__revision_file_t open logic.

* subversion/libsvn_fs_fs/rev_file.c
  (open_pack_or_rev_file): If we retry due to a potential background
                           pack, we should update the rev / pack file
                           base revision as well.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622946 13f79535-47bb-0310-9956-ffa450edef68