prune: honor --expire=never

prune: honor --expire=never

Previously, prune treated an expiration time of 0 to mean that no
expire argument was supplied, and everything should be pruned.  As a
result, "prune --expire=never" would prune all unreachable objects,
regardless of their timestamp.

prune can be called with --expire=never automatically by gc, when the
gc.pruneExpire configuration is set to "never".

Signed-off-by: Adam Simpkins <simpkins@facebook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>