checkout: avoid using the rev_info flag leak_pending

checkout: avoid using the rev_info flag leak_pending

The leak_pending flag is so awkward to use that multiple comments had to
be added around each occurrence.  We only use it for remembering the
commits whose marks we have to clear after checking if the old HEAD is
detached.  This is easy, though: We need to do that for the old commit,
the new one -- and for all refs.

Don't bother tracking exactly which commits need their flags cleared,
just nuke all we have in-core.  This change is safe because refs can
point at anything, so other program parts can't depend on any kept flags
anyway.  And since all refs are loaded we have to basically deal with
all commits anyway, so performance should not be negatively impacted.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>