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>

add-interactive.c

@@ -665,8 +665,16 @@ static int run_update(struct add_i_state *s, const struct pathspec *ps,
 
 	for (i = 0; i < files->items.nr; i++) {
 		const char *name = files->items.items[i].string;
-		if (files->selected[i] &&
-		    add_file_to_index(s->r->index, name, 0) < 0) {
+		struct stat st;
+
+		if (!files->selected[i])
+			continue;
+		if (lstat(name, &st) && is_missing_file_error(errno)) {
+			if (remove_file_from_index(s->r->index, name) < 0) {
+				res = error(_("could not stage '%s'"), name);
+				break;
+			}
+		} else if (add_file_to_index(s->r->index, name, 0) < 0) {
 			res = error(_("could not stage '%s'"), name);
 			break;
 		}

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