vim-patch:8.1.2281: 'showbreak' cannot be set for one window

Problem:    'showbreak' cannot be set for one window.
Solution:   Make 'showbreak' global-local.
ee85702c10

Change in oneleft() is N/A as the relevant condition was removed
(has_mbyte is always true for Nvim, so the condition was always false;
see commit 73dc9e9).

Use wp over curwin for curs_columns().

Required for v8.2.2903 (otherwise test fails as it'll leave the global
option set).

N/A patches for version.c:

vim-patch:8.1.2283: missed on use of p_sbr

Problem:    Missed on use of p_sbr.
Solution:   Add missing p_sbr change.
91e22eb6e0

Already ported in commit 43a874a.

src/nvim/charset.c

@@ -941,7 +941,7 @@ void getvcol(win_T *wp, pos_T *pos, colnr_T *start, colnr_T *cursor,
   // Also use this when 'list' is set but tabs take their normal size.
   if ((!wp->w_p_list || (wp->w_p_lcs_chars.tab1 != NUL))
       && !wp->w_p_lbr
-      && (*p_sbr == NUL)
+      && *get_showbreak_value(wp) == NUL
       && !wp->w_p_bri ) {
     for (;;) {
       head = 0;