allow running on a ref instead of a PR

When running from a PR, the ref is the head of the PR in the origin
repository. The PR is used to:

* display comments about the location of the destination PR
* asynchronously close / merge the destination PR when something
  happens in the origin PR

When only the ref is available, the destination PR must be closed and
the corresponding branch destroyed immediately after it concludes
because there is no convenient way to know what or when something
else will happen.

tests/origin-branch-fail/upgraded

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+set -ex
+
+destination_checkout="$1"
+destination_pr_json="$2"
+origin_checkout="$3"
+origin_ref="$4"
+
+test -d $destination_checkout
+test -d $origin_checkout
+test "$origin_ref"
+
+date +%s > $destination_checkout/last