Only format files changed in PR

[mdedetrich]

This is a nice to have feature, but basically the idea is that if you run this github action specifically on a PR it will only run scalafmt against the files that have been changed in the PR rather than the entire project

Scalafmt supports a -f flag which can be repeated with a list of files, i.e.

scalafmt -f file1 -f file2

There is a changed files action (see https://github.com/marketplace/actions/changed-files) which lets you get an output of all of the changed files which would then be fed into scalafmt. Then we can add a config for this, something like

formatOnlyChangedOnPullRequest: null // default behaviour
formatOnlyChangedOnPullRequest: file // most granular option, will strictly only format changed files
formatOnlyChangedOnPullRequest: dir // less granular, will format dirs that happen to have changed files

[mdedetrich]

Note that I made an issue on scalafmt upstream to support this feature as it may make more sense to implement it upstream rather than specifically in this project, see scalameta/scalafmt#3344