revision.c: --indexed-objects add objects from all worktrees

This is the result of single_worktree flag never being set (no way to up
until now). To get objects from current index only, set single_worktree.

The other add_index_objects_to_pending's caller is mark_reachable_objects()
(e.g. "git prune") which also mark objects from all indexes.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t5304-prune.sh b/t/t5304-prune.sh
index 133b584..cba45c7 100755
--- a/t/t5304-prune.sh
+++ b/t/t5304-prune.sh
@@ -283,4 +283,13 @@
 	git -C B prune
 '
 
+test_expect_success 'prune: handle index in multiple worktrees' '
+	git worktree add second-worktree &&
+	echo "new blob for second-worktree" >second-worktree/blob &&
+	git -C second-worktree add blob &&
+	git prune --expire=now &&
+	git -C second-worktree show :blob >actual &&
+	test_cmp second-worktree/blob actual
+'
+
 test_done