gc: rename keep_base_pack variable for --keep-largest-pack
As noted in an earlier change the keep_base_pack variable name is a
relic from an earlier on-list version of
ae4e89e549 ("gc: add
--keep-largest-pack option", 2018-04-15) before it was renamed to
--keep-largest-pack.
Let's change the variable name to avoid that confusion, it's easier to
read the code if there's a 1=1 mapping between the variable name and
option name.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>