unread [nfc]: Avoid computing unreadMessageIds twice.
unread [nfc]: Avoid computing unreadMessageIds twice.

This is simpler, and makes the `if (.. === ..) return input;`
optimization above make more sense.

It also does less computation, though the impact of that is probably
immaterial.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile