patch 8.0.1425: execute() does not work in completion of user command

Problem:    execute() does not work in completion of user command. (thinca)
Solution:   Switch off redir_off and restore it. (Ozaki Kiichi, closes #2492)
diff --git a/src/testdir/test_usercommands.vim b/src/testdir/test_usercommands.vim
index 9420321..21b1c4c 100644
--- a/src/testdir/test_usercommands.vim
+++ b/src/testdir/test_usercommands.vim
@@ -206,3 +206,15 @@
   com! -complete=customlist,CustomComp DoCmd :
   call assert_fails("call feedkeys(':DoCmd \<C-D>', 'tx')", 'E117:')
 endfunc
+
+func CallExecute(A, L, P)
+  " Drop first '\n'
+  return execute('echo "hi"')[1:]
+endfunc
+
+func Test_use_execute_in_completion()
+  command! -nargs=* -complete=custom,CallExecute DoExec :
+  call feedkeys(":DoExec \<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"DoExec hi', @:)
+  delcommand DoExec
+endfunc