More preparation on 'svnfsfs load-index': Move the revision format check
from the UI part into the logic part.

* subversion/svnfsfs/load-index-cmd.c
  (svn_fs_fs__load_index): Check that the revision does have an index.
  (load_index): Remove the check here.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632866 13f79535-47bb-0310-9956-ffa450edef68