completion: address sparse-checkout issues

Correct multiple issues with tab completion of the git sparse-checkout
command. These issues were:

1. git sparse-checkout <TAB> previously resulted in an incomplete list of
subcommands (it was missing reapply and add).
2. Subcommand options were not tab-completable.
3. git sparse-checkout set <TAB> and git sparse-checkout add <TAB> showed
both file names and directory names. While this may be a less surprising
behavior for non-cone mode, cone mode sparse checkouts should complete
only directory names.

Note that while the new strategy of just using git ls-tree to complete on
directory names is simple and a step in the right direction, it does have
some caveats. These are:

1. Likelihood of poor performance in large monorepos (as a result of
recursively completing directory names).
2. Inability to handle paths containing unusual characters.

These caveats will be fixed by subsequent commits in this series.

Signed-off-by: Lessley Dennington <lessleydennington@gmail.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2988,7 +2988,7 @@ _git_show_branch ()
 
 _git_sparse_checkout ()
 {
-	local subcommands="list init set disable"
+	local subcommands="list init set disable add reapply"
 	local subcommand="$(__git_find_on_cmdline "$subcommands")"
 	if [ -z "$subcommand" ]; then
 		__gitcomp "$subcommands"
@@ -2996,14 +2996,14 @@ _git_sparse_checkout ()
 	fi
 
 	case "$subcommand,$cur" in
-	init,--*)
-		__gitcomp "--cone"
-		;;
-	set,--*)
-		__gitcomp "--stdin"
-		;;
-	*)
+	*,--*)
+		__gitcomp_builtin sparse-checkout_$subcommand "" "--"
 		;;
+	set,*|add,*)
+		if [ "$(__git config core.sparseCheckoutCone)" == "true" ] ||
+		[ -n "$(__git_find_on_cmdline --cone)" ]; then
+			__gitcomp "$(git ls-tree -d -r HEAD --name-only)"
+		fi
 	esac
 }