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 2d95707
("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>

Author: newren

contrib/completion/git-completion.bash

@@ -3097,6 +3097,7 @@ _git_sparse_checkout ()
 {
 	local subcommands="list init set disable add reapply"
 	local subcommand="$(__git_find_on_cmdline "$subcommands")"
+	local using_cone=true
 	if [ -z "$subcommand" ]; then
 		__gitcomp "$subcommands"
 		return
@@ -3107,8 +3108,15 @@ _git_sparse_checkout ()
 		__gitcomp_builtin sparse-checkout_$subcommand "" "--"
 		;;
 	set,*|add,*)
-		if [ "$(__git config core.sparseCheckoutCone)" == "true" ] ||
-		[ -n "$(__git_find_on_cmdline --cone)" ]; then
+		if [[ "$(__git config core.sparseCheckout)" == "true" &&
+		      "$(__git config core.sparseCheckoutCone)" == "false" &&
+		      -z "$(__git_find_on_cmdline --cone)" ]]; then
+			using_cone=false
+		fi
+		if [[ -n "$(__git_find_on_cmdline --no-cone)" ]]; then
+			using_cone=false
+		fi
+		if [[ "$using_cone" == "true" ]]; then
 			__gitcomp_directories
 		fi
 	esac