checkout: clear commit marks after detached-orphan check

checkout: clear commit marks after detached-orphan check

When leaving a detached HEAD, we do a revision walk to make
sure the commit we are leaving isn't being orphaned.
However, this leaves crufty marks in the commit objects
which can confuse later walkers, like the one in
stat_tracking_info.

Let's clean up after ourselves to prevent this conflict.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>