builtin/gc: make `too_many_loose_objects()` reusable without GC config

builtin/gc: make `too_many_loose_objects()` reusable without GC config

To decide whether or not a repository needs to be repacked we estimate
the number of loose objects. If the number exceeds a certain threshold
we perform the repack, otherwise we don't.

This is done via `too_many_loose_objects()`, which takes as parameter
the `struct gc_config`. This configuration is only used to determine the
threshold. In a subsequent commit we'll add another caller of this
function that wants to pass a different limit than the one stored in
that structure.

Refactor the function accordingly so that we only take the limit as
parameter instead of the whole structure.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>