A broken reimplementation of Git could write an invalid index that
records both stage #0 and higher stage entries for the same path.
Notice and reject such an index, as there is no sensible fallback
(we do not know if the broken tool wanted to resolve and forgot to
remove higher stage entries, or if it wanted to unresolve and
forgot to remove the stage#0 entry).

* jp/index-with-corrupt-stages:
  read_index_unmerged(): remove unnecessary loop index adjustment
  read_index_from(): catch out of order entries when reading an index file