patch 8.0.0172: command line window does not work

Problem:    The command selected in the command line window is not executed.
            (Andrey Starodubtsev)
Solution:   Save and restore the command line at a lower level. (closes #1370)
diff --git a/src/testdir/test_history.vim b/src/testdir/test_history.vim
index 243ede7..ca31e3f 100644
--- a/src/testdir/test_history.vim
+++ b/src/testdir/test_history.vim
@@ -87,3 +87,20 @@
   call assert_equal(-1, histnr('abc'))
   call assert_fails('call histnr([])', 'E730:')
 endfunction
+
+function Test_Search_history_window()
+  new
+  call setline(1, ['a', 'b', 'a', 'b'])
+  1
+  call feedkeys("/a\<CR>", 'xt')
+  call assert_equal('a', getline('.'))
+  1
+  call feedkeys("/b\<CR>", 'xt')
+  call assert_equal('b', getline('.'))
+  1
+  " select the previous /a command
+  call feedkeys("q/kk\<CR>", 'x!')
+  call assert_equal('a', getline('.'))
+  call assert_equal('a', @/)
+  bwipe!
+endfunc