add --interactive: allow update to stage deleted files

The scripted version of `git add -i` used `git update-index --add
--remove`, but the built-in version implemented only the `--add` part.

This fixes https://github.com/msys2/MSYS2-packages/issues/3066

Reported-by: Christoph Reiter <reiter.christoph@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3701-add-interactive.sh

@@ -71,6 +71,15 @@ test_expect_success 'status works (commit)' '
 	grep "+1/-0 *+2/-0 file" output
 '
 
+test_expect_success 'update can stage deletions' '
+	>to-delete &&
+	git add to-delete &&
+	rm to-delete &&
+	test_write_lines u t "" | git add -i &&
+	git ls-files to-delete >output &&
+	test_must_be_empty output
+'
+
 test_expect_success 'setup expected' '
 	cat >expected <<-\EOF
 	index 180b47c..b6f2c08 100644