patch 8.2.3793: using "g:Func" as a funcref does not work in script context

Problem:    Using "g:Func" as a funcref does not work in script context
            because "g:" is dropped.
Solution:   Keep "g:" in the name.  Also add parenthesis to avoid confusing
            operator prececence. (closes #9336)

src/evalvars.c

@@ -1414,7 +1414,7 @@ ex_let_option(
 		n = (long)tv_get_number(tv);
 	}
 
-	if (opt_p_flags & P_FUNC && (tv->v_type == VAR_PARTIAL
+	if ((opt_p_flags & P_FUNC) && (tv->v_type == VAR_PARTIAL
 						|| tv->v_type == VAR_FUNC))
 	{
 	    // If the option can be set to a function reference or a lambda
@@ -2723,7 +2723,12 @@ eval_variable(
 		if (rettv != NULL)
 		{
 		    rettv->v_type = VAR_FUNC;
-		    rettv->vval.v_string = vim_strsave(ufunc->uf_name);
+		    if (STRNCMP(name, "g:", 2) == 0)
+			// Keep the "g:", otherwise script-local may be
+			// assumed.
+			rettv->vval.v_string = vim_strsave(name);
+		    else
+			rettv->vval.v_string = vim_strsave(ufunc->uf_name);
 		    if (rettv->vval.v_string != NULL)
 			func_ref(ufunc->uf_name);
 		}

src/testdir/test_vim9_func.vim

@@ -1224,6 +1224,25 @@ def Test_set_opfunc_to_lambda()
   CheckScriptSuccess(lines)
 enddef
 
+def Test_set_opfunc_to_global_function()
+  var lines =<< trim END
+    vim9script
+    def g:CountSpaces(type = ''): string
+      normal! '[V']y
+      g:result = getreg('"')->count(' ')
+      return ''
+    enddef
+    &operatorfunc = g:CountSpaces
+    new
+    'a b c d e'->setline(1)
+    feedkeys("g@_", 'x')
+    assert_equal(4, g:result)
+    bwipe!
+  END
+  CheckScriptSuccess(lines)
+  &operatorfunc = ''
+enddef
+
 def Test_lambda_type_allocated()
   # Check that unreferencing a partial using a lambda can use the variable type
   # after the lambda has been freed and does not leak memory.

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3793,
 /**/
     3792,
 /**/