Do not complete command switches after --

Recently, switch completion were given the menu behavior.
Unfortunately this breaks cases like

	:echo -- -mark<ret>

where the hypothetical user wanted to actually display "-mark", not
"-markup".

Simply bail if there is a double-dash. This is not fully correct,
for example it wrongly disables switch completion on

	echo -to-file -- -

but that's minor, we can fix it later.

In future, we should reuse the ParametersParser when computing completions,
which will obsolete this workaround.

src/command_manager.cc

@@ -808,7 +808,8 @@ Completions CommandManager::complete(const Context& context,
         const bool has_switches = not command.param_desc.switches.empty();
         auto is_switch = [=](StringView s) { return has_switches and s.substr(0_byte, 1_byte) == "-"; };
 
-        if (is_switch(token.content))
+        if (is_switch(token.content)
+            and not contains(tokens | drop(1) | transform(&Token::content), "--"))
         {
             auto switches = Kakoune::complete(token.content.substr(1_byte), pos_in_token,
                                               concatenated(command.param_desc.switches