patch 9.0.0036: 'fillchars' cannot have window-local values

Problem:    'fillchars' cannot have window-local values.
Solution:   Make 'fillchars' global-local. (closes #5206)
diff --git a/src/testdir/test_display.vim b/src/testdir/test_display.vim
index f5c8dc5..9fd0126 100644
--- a/src/testdir/test_display.vim
+++ b/src/testdir/test_display.vim
@@ -356,6 +356,34 @@
   set fillchars& fdc& foldmethod& foldenable&
 endfunc
 
+func Test_local_fillchars()
+  CheckScreendump
+
+  let lines =<< trim END
+      call setline(1, ['window 1']->repeat(3))
+      setlocal fillchars=stl:1,stlnc:a,vert:=,eob:x
+      vnew
+      call setline(1, ['window 2']->repeat(3))
+      setlocal fillchars=stl:2,stlnc:b,vert:+,eob:y
+      new
+      wincmd J
+      call setline(1, ['window 3']->repeat(3))
+      setlocal fillchars=stl:3,stlnc:c,vert:<,eob:z
+      vnew
+      call setline(1, ['window 4']->repeat(3))
+      setlocal fillchars=stl:4,stlnc:d,vert:>,eob:o
+  END
+  call writefile(lines, 'Xdisplayfillchars')
+  let buf = RunVimInTerminal('-S Xdisplayfillchars', #{rows: 12})
+  call VerifyScreenDump(buf, 'Test_display_fillchars_1', {})
+
+  call term_sendkeys(buf, ":wincmd k\r")
+  call VerifyScreenDump(buf, 'Test_display_fillchars_2', {})
+
+  call StopVimInTerminal(buf)
+  call delete('Xdisplayfillchars')
+endfunc
+
 func Test_display_linebreak_breakat()
   new
   vert resize 25