patch 8.0.1206: no autocmd for entering or leaving the command line

Problem:    No autocmd for entering or leaving the command line.
Solution:   Add CmdlineEnter and CmdlineLeave.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index f5a2289..6af0820 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -793,3 +793,25 @@
   bwipe Xfoo
   bwipe Xbar
 endfunc
+
+func Test_Cmdline()
+  au! CmdlineEnter : let g:entered = expand('<afile>')
+  au! CmdlineLeave : let g:left = expand('<afile>')
+  let g:entered = 0
+  let g:left = 0
+  call feedkeys(":echo 'hello'\<CR>", 'xt')
+  call assert_equal(':', g:entered)
+  call assert_equal(':', g:left)
+  au! CmdlineEnter
+  au! CmdlineLeave
+
+  au! CmdlineEnter / let g:entered = expand('<afile>')
+  au! CmdlineLeave / let g:left = expand('<afile>')
+  let g:entered = 0
+  let g:left = 0
+  call feedkeys("/hello<CR>", 'xt')
+  call assert_equal('/', g:entered)
+  call assert_equal('/', g:left)
+  au! CmdlineEnter
+  au! CmdlineLeave
+endfunc