Feature request: pull_request base branch change event

[stefanfoulis]

Currently it is possible to react to the event when a different base branch is assigned to a PR with the edited event like so:

on:
  pull_request:
    types: [edited]

However this event also fires if the PR title or description changes. And it does not fire if new commits are added to the base branch.

My suggestion is to add a new event, called something like base_changed, that will fire if a different base branch is assigned to the PR or if the head of the base branch changes.

on:
  pull_request:
    types: [base_changed]

In my case I'd like to use it to re-run CI on a PR using the merge commit if the base changes. I know there are github actions that attempt this, but they all have to resort to workarounds to achieve this. E.g by adding an empty commit to the PR. And they need a Personal Access Token to work.
Another workaround could be to use workflow_dispatch or workflow_run and put all the CI logic in those. But when dispatching that shared workflow from various events we loose the "PR context" and all the associated automatic commit statuses. Overall these other approaches are all very cumbersome.

[willbush]

This would be useful for an action that runs on every PR in nixpkgs. more context: NixOS/nixpkgs#313345

[JohnRTitor]

Would love to get this implemented.