patch 8.1.0037: cannot easily append lines to another buffer

Problem:    Cannot easily append lines to another buffer.
Solution:   Add appendbufline().
diff --git a/src/testdir/test_edit.vim b/src/testdir/test_edit.vim
index 3af8c4c..ab2fe7c 100644
--- a/src/testdir/test_edit.vim
+++ b/src/testdir/test_edit.vim
@@ -527,7 +527,7 @@
   " Tab in completion mode
   let path=expand("%:p:h")
   new
-  call setline(1, [path."/", ''])
+  call setline(1, [path. "/", ''])
   call feedkeys("Arunt\<c-x>\<c-f>\<tab>\<cr>\<esc>", 'tnix')
   call assert_match('runtest\.vim', getline(1))
   %d