Support `pull_request` events

When this is triggered by the `pull_request` events, the shallow fetch depth often results in the error `fatal: path 'package.json' exists on disk, but not in '[commit hash]'` because the `before` commit isn't present.

We should update this action to include documentation on how to avoid this scenario, and support for doing the right thing in this scenario.

See here for more details: https://github.com/MetaMask/controllers/pull/981#issuecomment-1320660129

The issue names no repository files or tests. Start by tracing how the action handles pull_request events and shallow fetches, then review the linked MetaMask/controllers discussion for the failure context; done means the scenario is supported and the avoidance guidance is documented.

Written by the indexing model from the issue text.