patch 8.2.1571: Vim9: count() third argument cannot be "true"

Problem:    Vim9: count() third argument cannot be "true".
Solution:   use tv_get_bool_chk(). (closes #6818)

src/list.c

@@ -2167,7 +2167,7 @@ f_count(typval_T *argvars, typval_T *rettv)
     int		error = FALSE;
 
     if (argvars[2].v_type != VAR_UNKNOWN)
-	ic = (int)tv_get_number_chk(&argvars[2], &error);
+	ic = (int)tv_get_bool_chk(&argvars[2], &error);
 
     if (argvars[0].v_type == VAR_STRING)
     {

src/testdir/test_vim9_func.vim

@@ -1492,6 +1492,11 @@ def Fibonacci(n: number): number
   endif
 enddef
 
+def Test_count()
+  assert_equal(3, count('ABC ABC ABC', 'b', true))
+  assert_equal(0, count('ABC ABC ABC', 'b', false))
+enddef
+
 def Test_recursive_call()
   assert_equal(6765, Fibonacci(20))
 enddef

src/typval.c

@@ -283,7 +283,6 @@ tv_get_bool(typval_T *varp)
 tv_get_bool_chk(typval_T *varp, int *denote)
 {
     return tv_get_bool_or_number_chk(varp, denote, TRUE);
-
 }
 
 #ifdef FEAT_FLOAT

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1571,
 /**/
     1570,
 /**/