"Add Repo" dialog lets you add existing repo

To reproduce, copy the URL of any repo listed in ManageRepos then add that as a new repo. It will add it. Also, it will add a duplicate repo if the repo URL comes in via an Intent. I tried to get to the bottom of it, but it stumped me for today.

Oh yeah, so it does. I'll fix that up.

Status changed to closed

Fixed. The process is a lot more robust now, and more importantly, it doesn't just deal with repos coming from an intent, but also with repos being added form the Manage Repos activity.

There are two outstanding issues, which are of lower priority but should still be done. I'll create issues for them and link here so that they don't get forgotten.