patch 8.2.3608: users who type "q:" instead of ":q" are confused

Problem:    Users who type "q:" instead of ":q" are confused.
Solution:   Add an autocmd to give a message that explains this is the
            command-line window. (Egor Zvorykin, closes #9146)

src/testdir/test_autocmd.vim

@@ -2458,6 +2458,7 @@ func Test_autocmd_CmdWinEnter()
   CheckFeature cmdwin
 
   let lines =<< trim END
+    augroup vimHints | au! | augroup END
     let b:dummy_var = 'This is a dummy'
     autocmd CmdWinEnter * quit
     let winnr = winnr('$')

src/testdir/test_cmdline.vim

@@ -1255,6 +1255,7 @@ func Test_cmdwin_restore()
   CheckScreendump
 
   let lines =<< trim [SCRIPT]
+    augroup vimHints | au! | augroup END
     call setline(1, range(30))
     2split
   [SCRIPT]

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3608,
 /**/
     3607,
 /**/