patch 9.0.1550: in cmdline window S-Tab does not select previous completion

Problem:    In cmdline window S-Tab does not select previous completion.
            (Maxim Kim)
Solution:   Add a mappint for S-Tab. (closes #12116)
diff --git a/src/testdir/test_cmdwin.vim b/src/testdir/test_cmdwin.vim
index ad05f3e..e0b5833 100644
--- a/src/testdir/test_cmdwin.vim
+++ b/src/testdir/test_cmdwin.vim
@@ -359,6 +359,22 @@
   set wildmenu& wildchar&
 endfunc
 
+func Test_cmdwin_cmd_completion()
+  set wildmenu wildchar=<Tab>
+  com! -nargs=* -complete=command SomeOne echo 'one'
+  com! -nargs=* -complete=command SomeTwo echo 'two'
+  call feedkeys("q:aSome\<Tab>\<Home>\"\<CR>", 'tx')
+  call assert_equal('"SomeOne', @:)
+  call feedkeys("q:aSome\<Tab>\<Tab>\<Home>\"\<CR>", 'tx')
+  call assert_equal('"SomeTwo', @:)
+  call feedkeys("q:aSome\<Tab>\<Tab>\<S-Tab>\<Home>\"\<CR>", 'tx')
+  call assert_equal('"SomeOne', @:)
+
+  delcom SomeOne
+  delcom SomeTwo
+  set wildmenu& wildchar&
+endfunc
+
 func Test_cmdwin_ctrl_bsl()
   " Using CTRL-\ CTRL-N in cmd window should close the window
   call feedkeys("q:\<C-\>\<C-N>", 'xt')