A run triggered by a pull_request-event workflow carries a #<n> pseudo-ref
in prettyref rather than a real branch name. Dispatching a workflow with
that as the ref 500s. resolve_run now recognizes the #<n> form and falls
back to the same branch_for_pr lookup --pr already uses, so ci-rerun --run
works on PR-triggered runs the same way ci-rerun --pr does.
diagnosis + discriminating control table (real branch vs #<n> pseudo-ref)
from atlas.
fixes#4201
7b9d8e4c4c