completion: fix logic for determining whether cone mode is active

_git_sparse_checkout() was checking whether we were in cone mode by checking whether either: A) core.sparseCheckoutCone was "true" B) "--cone" was specified on the command line This code has 2 bugs I didn't catch in my review at the time 1) core.sparseCheckout must be "true" for core.sparseCheckoutCone to be relevant (which matters since "git sparse-checkout disable" only unsets core.sparseCheckout, not core.sparseCheckoutCone) 2) The presence of "--no-cone" should override any config setting Further, I forgot to update this logic as part of 2d95707a02 ("sparse-checkout: make --cone the default", 2022-04-22) for the new default. Update the code for the new default and make it be more careful in determining whether to complete based on cone mode or non-cone mode. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>