builtin-gc.c: allow disabling all auto-gc'ing by assigning 0 to gc.auto

builtin-gc.c: allow disabling all auto-gc'ing by assigning 0 to gc.auto

The gc.auto configuration variable is somewhat ambiguous now that there
is also a gc.autopacklimit setting. Some users may assume that it controls
all auto-gc'ing. Also, now users must set two configuration variables to
zero when they want to disable autopacking. Since it is unlikely that users
will want to autopack based on some threshold of pack files when they have
disabled autopacking based on the number of loose objects, be nice and allow
a setting of zero for gc.auto to disable all autopacking.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>