Follow issue references to other repositories

[rick]

Currently we build a graph including all issues encountered as we iterate through a repository's issue collection. If we encounter a depends/blocks reference to an issue in another repository we currently do not fetch that issue's information from the other repository and so our behavior in graph construction is less than ideal (I think we end up having a disconnected node floating around in the graph, with no title information? It may be worse -- maybe we crash, not sure).

Anyway, we're now basically doing one layer of a breadth-first search. What we need to do is track the horizon of referenced but unseen issues and do a full breadth-first search until that horizon list is empty.

Some edge cases that we need to handle (I mean, we probably already encounter these and handle them poorly):

• an issue is referenced which does not exist
• an issue is referenced which we cannot view (perhaps we can't view the repository that contains it)