Fix checkout in Pronto on external pull requests
Fix checkout in Pronto on external pull requests

Since the target branch was in a different repository, the action failed
since it couldn't find the reference.

The code here is based on a recent change in Pronto [1] and with a comparison
between the repo.url property of pull_request.head and pull_request.base
to determine if the pull request was created from a forked repository

[1] https://github.com/prontolabs/pronto/commit/4fe28418b6

consul/consul,consul/consul,consul/consul,consul/consul,consul/consul