use PR head sha by default for pull_request and pull_request_target

[bryangingechen]

Proposed changes

As explained in the README, when triggered via the pull_request (or pull_request_target) events, this workflow will not find the correct PR for the first commit. However, there is a workaround of including github.event.pull_request.head.sha as a config option. Here we simply use that as the default sha if the event is a pull_request or pull_request_target, which should save the user from having to provide some configuration.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...