patch 8.2.1590: Vim9: bufnr() doesn't take "true" argument

Problem:    Vim9: bufnr() doesn't take "true" argument.
Solution:   use tv_get_bool_chk(). (closes #6863)

src/evalbuffer.c

@@ -391,7 +391,7 @@ f_bufnr(typval_T *argvars, typval_T *rettv)
     // new buffer.
     if (buf == NULL
 	    && argvars[1].v_type != VAR_UNKNOWN
-	    && tv_get_number_chk(&argvars[1], &error) != 0
+	    && tv_get_bool_chk(&argvars[1], &error) != 0
 	    && !error
 	    && (name = tv_get_string_chk(&argvars[0])) != NULL
 	    && !error)

src/testdir/test_vim9_func.vim

@@ -1327,6 +1327,10 @@ enddef
 def Test_bufnr()
   let buf = bufnr()
   assert_equal(buf, bufnr('%'))
+
+  buf = bufnr('Xdummy', true)
+  assert_notequal(-1, buf)
+  exe 'bwipe! ' .. buf
 enddef
 
 def Test_col()

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1590,
 /**/
     1589,
 /**/