fish.rs: fix regression in fish_xdm_login_hack_hack_hack_hack

This is off by one from the C++ version.

It wasn't super obvious why this worked in the first place.
Looks like args[0] is "-" because we are invoked like

    fish -c 'exec "${@}"' - "${@}"

and it looks like "-" is treated like "--" by bash, so we emulate that.
See https://github.com/fish-shell/fish-shell/issues/367#issuecomment-11740812

fish-rust/src/fish.rs

@@ -856,7 +856,7 @@ fn fish_xdm_login_hack_hack_hack_hack(cmds: &mut Vec<OsString>, args: &[&wstr])
         // We're going to construct a new command that starts with exec, and then has the
         // remaining arguments escaped.
         let mut new_cmd = OsString::from("exec");
-        for arg in args {
+        for arg in &args[1..] {
             new_cmd.push(" ");
             new_cmd.push(escape_single_quoted_hack_hack_hack_hack(arg));
         }