repo_read_index: don't discard the index

repo_read_index: don't discard the index

Have 'repo_read_index()' behave more like the other read_index family of
functions and don't discard the index if it has already been populated
and instead rely on the quick return of read_index_from which has:

  /* istate->initialized covers both .git/index and .git/sharedindex.xxx */
  if (istate->initialized)
    return istate->cache_nr;

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>