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)

src/evalfunc.c

@@ -2886,6 +2886,7 @@ f_execute(typval_T *argvars, typval_T *rettv)
     int		save_emsg_silent = emsg_silent;
     int		save_emsg_noredir = emsg_noredir;
     int		save_redir_execute = redir_execute;
+    int		save_redir_off = redir_off;
     garray_T	save_ga;
 
     rettv->vval.v_string = NULL;
@@ -2928,6 +2929,7 @@ f_execute(typval_T *argvars, typval_T *rettv)
 	save_ga = redir_execute_ga;
     ga_init2(&redir_execute_ga, (int)sizeof(char), 500);
     redir_execute = TRUE;
+    redir_off = FALSE;
 
     if (cmd != NULL)
 	do_cmdline_cmd(cmd);
@@ -2958,6 +2960,7 @@ f_execute(typval_T *argvars, typval_T *rettv)
     redir_execute = save_redir_execute;
     if (redir_execute)
 	redir_execute_ga = save_ga;
+    redir_off = save_redir_off;
 
     /* "silent reg" or "silent echo x" leaves msg_col somewhere in the
      * line.  Put it back in the first column. */

src/testdir/test_usercommands.vim

@@ -206,3 +206,15 @@ func Test_CmdCompletion()
   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

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1425,
 /**/
     1424,
 /**/