patch 8.2.2569: 'fillchars' "stl" and "stlnc" items must be single byte

Problem:    'fillchars' "stl" and "stlnc" items must be single byte.
Solution:   Accept multi-byte characters. (Christian Wellenbrock, Yegappan
            Lakshmanan, closes #7927)
diff --git a/src/testdir/test_statusline.vim b/src/testdir/test_statusline.vim
index b28dbb9..f6c05f6 100644
--- a/src/testdir/test_statusline.vim
+++ b/src/testdir/test_statusline.vim
@@ -464,5 +464,20 @@
   set ls& stl&
 endfunc
 
+" Test using a multibyte character for 'stl' and 'stlnc' items in 'fillchars'
+" with a custom 'statusline'
+func Test_statusline_mbyte_fillchar()
+  only
+  set laststatus=2
+  set fillchars=vert:\|,fold:-,stl:━,stlnc:═
+  set statusline=a%=b
+  call assert_match('^a\+━\+b$', s:get_statusline())
+  vnew
+  call assert_match('^a\+━\+b━a\+═\+b$', s:get_statusline())
+  wincmd w
+  call assert_match('^a\+═\+b═a\+━\+b$', s:get_statusline())
+  set statusline& fillchars& laststatus&
+  %bw!
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab