vim-patch:8.1.0449: fix display of 'rnu' with folded lines #9481

Problem:    When 'rnu' is set folded lines are not displayed correctly.
            (Vitaly Yashin)
Solution:   When only redrawing line numbers do draw folded lines.
            (closes vim/vim#3484)
https://github.com/vim/vim/commit/7701f308565fdc7b5096a6597d9c3b63de0bbcec

---
Explanation:
Before this patch, relative line numbers would update on a cursor
movement and overwrite fold highlighting in the line number columns.
Other operations can cause the fold highlighting to overwrite the line
number styles. Together, this causes the highlighting in the line number
columns to flicker back and forth while editing.

Test case: create `t.vim` with these contents:

    set fdm=marker rnu foldcolumn=2
    call setline(1, ["{{{1", "nline 1", "{{{1", "line 2"])

and then call `nvim -u NORC -S t.vim` and press `j`; observe that the fold
highlighting disappears.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index 27d72ffbf7..56ca4ad4b6 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -1409,7 +1409,13 @@ static void win_update(win_T *wp)
       if (wp->w_p_rnu) {
         // 'relativenumber' set: The text doesn't need to be drawn, but
         // the number column nearly always does.
-        (void)win_line(wp, lnum, srow, wp->w_grid.Rows, true, true);
+        fold_count = foldedCount(wp, lnum, &win_foldinfo);
+        if (fold_count != 0) {
+          fold_line(wp, fold_count, &win_foldinfo, lnum, row);
+          --fold_count;
+        } else {
+          (void)win_line(wp, lnum, srow, wp->w_grid.Rows, true, true);
+        }
       }
 
       // This line does not need to be drawn, advance to the next one.