Allow specific repositories to be used for cross repository quick search

At the moment the quick search uses either all the repos in an instance, or a single repo. The problem with that is that if you want to search from something over multiple repos (but not all) there is a good chance you will hit the 5 second limit and perhaps miss out on the results you actually want.

It would be good if you could specify which repositories are to be to be searched in a cross repo search.

As an extension, it would also be beneficial if there is also an "exclude" option, so users can search all repositories except a few specified.