patch 8.0.0656: cannot use ! after some user commands

Problem:    Cannot use ! after some user commands.
Solution:   Properly check for existing command. (Higashi Higashi)

src/ex_docmd.c

@@ -2370,7 +2370,8 @@ do_one_cmd(
 	goto doend;
     }
     /* Check for wrong commands. */
-    if (*p == '!' && ea.cmd[1] == 0151 && ea.cmd[0] == 78)
+    if (*p == '!' && ea.cmd[1] == 0151 && ea.cmd[0] == 78
+	    && !IS_USER_CMDIDX(ea.cmdidx))
     {
 	errormsg = uc_fun_cmd();
 	goto doend;

src/testdir/test_vimscript.vim

@@ -1400,6 +1400,15 @@ func Test_delfunction_force()
     delfunc! Xtest
 endfunc
 
+" Test using bang after user command				    {{{1
+func Test_user_command_with_bang()
+    command -bang Nieuw let nieuw = 1
+    Ni!
+    call assert_equal(1, nieuw)
+    unlet nieuw
+    delcommand Nieuw
+endfunc
+
 "-------------------------------------------------------------------------------
 " Modelines								    {{{1
 " vim: ts=8 sw=4 tw=80 fdm=marker

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    656,
 /**/
     655,
 /**/