completion: fix logic for determining whether cone mode is active

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>