Support files property in git diff between two commits

[keyonghan]

The current compareCommits does git diff between two commits, but returns only limited comparison results:
https://github.com/SpinlockLabs/github.dart/blob/master/lib/src/common/model/repos.dart#L9-L14

Based on api: https://docs.github.com/en/rest/reference/repos#compare-two-commits, files property plays an important role in determining what files are changed. However it is missing in the above compareCommits function.

Our use case is git diff between two commits and need to know if one is the revert of the other.

[github-actions]

👋 Thanks for reporting! @robrbecker will take a look.