vim-patch:8.0.0735: no indication that the quickfix window/buffer changed

Problem:    There is no way to notice that the quickfix window contents has
            changed.
Solution:   Increment b:changedtick when updating the quickfix window.
            (Yegappan Lakshmanan)
a8788f4d0b

runtime/doc/quickfix.txt

@@ -423,7 +423,9 @@ EXECUTE A COMMAND IN ALL THE BUFFERS IN QUICKFIX OR LOCATION LIST:
 			which will indicate the command that produced the
 			quickfix list. This can be used to compose a custom
 			status line if the value of 'statusline' is adjusted
-			properly.
+			properly. Whenever this buffer is modified by a
+			quickfix command or function, the |b:changedtick|
+			variable is incremented.
 
 							*:lop* *:lopen*
 :lop[en] [height]	Open a window to show the location list for the

src/nvim/quickfix.c

@@ -2887,6 +2887,7 @@ static void qf_update_buffer(qf_info_T *qi, qfline_T *old_last)
     qf_update_win_titlevar(qi);
 
     qf_fill_buffer(qi, buf, old_last);
+    buf_inc_changedtick(buf);
 
     if (old_last == NULL) {
       (void)qf_win_pos_update(qi, 0);

src/nvim/testdir/test_quickfix.vim

@@ -2245,3 +2245,27 @@ func Test_resize_from_copen()
 	augroup! QF_Test
     endtry
 endfunc
+
+" Tests for the quickfix buffer b:changedtick variable
+func Xchangedtick_tests(cchar)
+  call s:setup_commands(a:cchar)
+
+  new | only
+
+  Xexpr "" | Xexpr "" | Xexpr ""
+
+  Xopen
+  Xolder
+  Xolder
+  Xaddexpr "F1:10:Line10"
+  Xaddexpr "F2:20:Line20"
+  call g:Xsetlist([{"filename":"F3", "lnum":30, "text":"Line30"}], 'a')
+  call g:Xsetlist([], 'f')
+  call assert_equal(8, getbufvar('%', 'changedtick'))
+  Xclose
+endfunc
+
+func Test_changedtick()
+    call Xchangedtick_tests('c')
+    call Xchangedtick_tests('l')
+endfunc