Make Check Changelog compatible with actions/checkout v2 (#1229)
Make Check Changelog compatible with actions/checkout v2 (#1229)

The GitHub action `actions/checkout` now only shallow clones the Git
repo as of v2+, meaning the Check Changelog action fails with:
`fatal: ambiguous argument 'remotes/origin/main': unknown revision or path not in the working tree.`

To fix, an explicit `git fetch` has been added of the HEAD of the default branch.

In addition, the event types on which the check is run has been updated
to include missing event types:
https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request

Refs GUS-W-9728468.
heroku/heroku-buildpack-python,Scalingo/python-buildpack,Scalingo/python-buildpack,heroku/heroku-buildpack-python