completion: improve sparse-checkout cone mode directory completion

Use new __gitcomp_directories method to complete directory names in cone
mode sparse-checkouts. This method addresses the caveat of poor
performance in monorepos from the previous commit (by completing only one
level of directories).

The unusual character caveat from the previous commit will be fixed by the
final commit in this series.

Co-authored-by: Elijah Newren <newren@gmail.com>
Co-authored-by: Lessley Dennington <lessleydennington@gmail.com>
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

@@ -2986,6 +2986,36 @@ _git_show_branch ()
 	__git_complete_revlist
 }
 
+__gitcomp_directories ()
+{
+	local _tmp_dir _tmp_completions
+
+	# Get the directory of the current token; this differs from dirname
+	# in that it keeps up to the final trailing slash.  If no slash found
+	# that's fine too.
+	[[ "$cur" =~ .*/ ]]
+	_tmp_dir=$BASH_REMATCH
+
+	# Find possible directory completions, adding trailing '/' characters
+	_tmp_completions="$(git ls-tree -d --name-only HEAD $_tmp_dir |
+		sed -e s%$%/%)"
+
+	if [[ -n "$_tmp_completions" ]]; then
+		# There were some directory completions, so find ones that
+		# start with "$cur", the current token, and put those in COMPREPLY
+		local i=0 c IFS=$' \t\n'
+		for c in $_tmp_completions; do
+		if [[ $c == "$cur"* ]]; then
+			COMPREPLY+=("$c")
+		fi
+		done
+	elif [[ "$cur" =~ /$ ]]; then
+		# No possible further completions any deeper, so assume we're at
+		# a leaf directory and just consider it complete
+		__gitcomp_direct_append "$cur "
+	fi
+}
+
 _git_sparse_checkout ()
 {
 	local subcommands="list init set disable add reapply"
@@ -3002,7 +3032,7 @@ _git_sparse_checkout ()
 	set,*|add,*)
 		if [ "$(__git config core.sparseCheckoutCone)" == "true" ] ||
 		[ -n "$(__git_find_on_cmdline --cone)" ]; then
-			__gitcomp "$(git ls-tree -d -r HEAD --name-only)"
+			__gitcomp_directories
 		fi
 	esac
 }

t/t9902-completion.sh

@@ -1477,21 +1477,30 @@ test_expect_success 'cone mode sparse-checkout completes directory names' '
 	(
 		cd sparse-checkout &&
 		test_completion "git sparse-checkout set f" <<-\EOF
-		folder1 Z
-		folder1/0 Z
-		folder1/0/1 Z
-		folder2 Z
-		folder2/0 Z
-		folder3 Z
+		folder1/
+		folder2/
+		folder3/
+		EOF
+	) &&
+
+	(
+		cd sparse-checkout &&
+		test_completion "git sparse-checkout set folder1/" <<-\EOF
+		folder1/0/
+		EOF
+	) &&
+
+	(
+		cd sparse-checkout &&
+		test_completion "git sparse-checkout set folder1/0/" <<-\EOF
+		folder1/0/1/
 		EOF
 	) &&
 
 	(
 		cd sparse-checkout/folder1 &&
-		test_completion "git sparse-checkout add " <<-\EOF
-		./ Z
-		0 Z
-		0/1 Z
+		test_completion "git sparse-checkout add 0" <<-\EOF
+		0/
 		EOF
 	)
 '
@@ -1517,12 +1526,9 @@ test_expect_success 'git sparse-checkout set --cone completes directory names' '
 	(
 		cd sparse-checkout &&
 		test_completion "git sparse-checkout set --cone f" <<-\EOF
-		folder1 Z
-		folder1/0 Z
-		folder1/0/1 Z
-		folder2 Z
-		folder2/0 Z
-		folder3 Z
+		folder1/
+		folder2/
+		folder3/
 		EOF
 	)
 '