ls-files: don't needlessly pass around stage variable

ls-files: don't needlessly pass around stage variable

Now that read_tree() has been moved to ls-files.c we can get rid of
the stage != 1 case that'll never happen.

Let's not use read_tree_recursive() as a pass-through to pass "stage =
1" either. For now we'll pass an unused "stage = 0" for consistency
with other read_tree_recursive() callers, that argument will be
removed in a follow-up commit.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>