don't show the follow-up link if already following up

This patch adds a small check in hook_link to skip the follow up link if the user is already on the follow up page. I've seen a number of people actually click that link half-way through replying to an issue. Having the link there seems like an endless loop as far as the interface is concerned.

I'm not 100% sure you'll want to make this change, but here it is anyway.