receive-pack: drop unused "commands" from prepare_shallow_update()

receive-pack: drop unused "commands" from prepare_shallow_update()

We pass in the list of proposed ref updates to prepare_shallow_update(),
but that function doesn't actually need it (and never has since its
inception in 0a1bc12b6e4). Only its caller, update_shallow_info(), needs
to look at the command list.

Let's drop the unused parameter to reduce confusion.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>