gc: add gc.bigPackThreshold config

gc: add gc.bigPackThreshold config

The --keep-largest-pack option is not very convenient to use because
you need to tell gc to do this explicitly (and probably on just a few
large repos).

Add a config key that enables this mode when packs larger than a limit
are found. Note that there's a slight behavior difference compared to
--keep-largest-pack: all packs larger than the threshold are kept, not
just the largest one.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>