patch 8.1.0449: when 'rnu' is set folded lines are not displayed correctly

Problem:    When 'rnu' is set folded lines are not displayed correctly.
            (Vitaly Yashin)
Solution:   When only redrawing line numbers do draw folded lines.
            (closes #3484)
diff --git a/src/testdir/test_fold.vim b/src/testdir/test_fold.vim
index df4b12c..0384b4f 100644
--- a/src/testdir/test_fold.vim
+++ b/src/testdir/test_fold.vim
@@ -1,6 +1,7 @@
 " Test for folding
 
 source view_util.vim
+source screendump.vim
 
 func PrepIndent(arg)
   return [a:arg] + repeat(["\t".a:arg], 5)
@@ -674,3 +675,23 @@
   set fdm&
   enew!
 endfunc
+
+func Test_folds_with_rnu()
+  if !CanRunVimInTerminal()
+    return
+  endif
+
+  call writefile([
+	\ 'set fdm=marker rnu foldcolumn=2',
+	\ 'call setline(1, ["{{{1", "nline 1", "{{{1", "line 2"])',
+	\ ], 'Xtest_folds_with_rnu')
+  let buf = RunVimInTerminal('-S Xtest_folds_with_rnu', {})
+
+  call VerifyScreenDump(buf, 'Test_folds_with_rnu_01', {})
+  call term_sendkeys(buf, "j")
+  call VerifyScreenDump(buf, 'Test_folds_with_rnu_02', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('Xtest_folds_with_rnu')
+endfunc