Only lint modified files

[samuelmeuli]

Currently, all files in a repository are being linted. The action should detect which files are part of the relevant commit/PR and only check these files.

[samuelmeuli]

Unfortunately, there currently doesn't seem to be a simple way to implement this feature.

The problem is that the GitHub checks API only allows annotating commits, not pull requests. And if this feature were implemented for commits (only modified files in a commit would be linted/annotated), it would result in weird behavior on pull requests: If a user pushed another commit to an existing PR, only the files modified in that commit would be annotated. And because the GitHub UI only shows annotations for the most recent commit on a PR, the annotations would be incomplete.