upload-pack: use priority queue in reachable() check

upload-pack: use priority queue in reachable() check

Like a lot of old commit-traversal code, this keeps a
commit_list in commit-date order, and and inserts parents
into the list. This means each insertion is potentially
linear, and the whole thing is quadratic (though the exact
runtime depends on the relationship between the commit dates
and the parent topology).

These days we have a priority queue, which can do the same
thing with a much better worst-case time.

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