This is for symmetry with `issue list`.
The problem is that the `Repository.pullRequests` connection doesn't
support filtering by assignee, therefore we need to switch to search API
in case an assignee was specified. This is awkward, but I don't see
another way.
When on a `patch-1` branch locally, `gh pr view` would happily open the
first open PR it finds with "patch-1" as its head, even those coming
from forks.
