ref-filter: clear reachable list pointers after freeing

ref-filter: clear reachable list pointers after freeing

In `reach_filter()`, we pop all commits from the reachable lists,
leaving them empty. But because we're operating on a list pointer that
was passed by value, the original `filter.reachable_from` and
`filter.unreachable_from` pointers are left dangling.

As is the case with the previous commit, nobody touches either of these
fields after calling `reach_filter()`, so leaving them dangling is OK.
But this future proofs against dangerous situations.

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