Backport: Handle unread-command-events consistently (bug#23980)

* src/keyboard.c (read_char): Events put into `unread-command-events'
with the form (t . EVENT) should always have the t stripped when read
out.
* test/src/keyboard-tests.el: New tests for `unread-command-events'.

(cherry picked from commit 1f3f4b12)

src/keyboard.c

@@ -2809,6 +2809,9 @@ read_char (int commandflag, Lisp_Object map,
 
       if (EQ (c, make_number (-2)))
 	return c;
+
+      if (CONSP (c) && EQ (XCAR (c), Qt))
+	c = XCDR (c);
   }
 
  non_reread:

test/src/keyboard-tests.el

+;;; keyboard-tests.el --- Tests for keyboard.c -*- lexical-binding: t -*-
+
+;; Copyright 2017 Free Software Foundation, Inc.
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+
+(ert-deftest keyboard-unread-command-events ()
+  "Test `unread-command-events'."
+  (should (equal (progn (push ?\C-a unread-command-events)
+                        (read-event nil nil 1))
+                 ?\C-a))
+  (should (equal (progn (run-with-timer
+                         1 nil
+                         (lambda () (push '(t . ?\C-b) unread-command-events)))
+                        (read-event nil nil 2))
+                 ?\C-b)))
+
+(provide 'keyboard-tests)
+;;; keyboard-tests.el ends here