Allow viewing a PR when its origin (fork or branch) is gone
This will ensure a pull-request remains accessible despite its source
being gone by relying on the newly introduced reference for each PR.
Since that reference is in the main repo and is not going away when the
fork or branch gets deleted, it is just as easy to rely on it when the
source of the PR disappears.
Fixes https://pagure.io/pagure/issue/2492
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>