GitHub Actions are triggered by webhook events. The original payload of the event is stored in a file that actions can read at workflow/event.json (see the docs). We’ve made a slight change to the payload for push events in Actions. The following attributes have been removed from the commits section of push payloads for Actions:
addedremovedmodified
These diffs can instead be requested using the API https://developer.github.com/v3/repos/commits/#get-a-single-commit.