Make default expiration period of reflog used for stash infinite

This makes the default expiration period for the reflog that implements
stash infinite.

The original behaviour to autoexpire old stashes can be restored by using
the gc.refs/stash.{reflogexpire,reflogexpireunreachable} configration
variables introduced by the previous commit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>