builtin/gc: forward git-gc(1)'s `--auto` flag when packing refs

builtin/gc: forward git-gc(1)'s `--auto` flag when packing refs

Forward the `--auto` flag to git-pack-refs(1) when it has been invoked
with this flag itself. This does not change anything for the "files"
backend, which will continue to eagerly pack refs. But it does ensure
that the "reftable" backend only compacts refs as required.

This change does not impact git-maintenance(1) because this command will
in fact never run the pack-refs task when run with `--auto`. This issue
will be addressed in a subsequent commit.

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