If you have 2 diverged branches of the same original repo and you want to identify the differences e.g. to merge the code:
Best one:
https://stackoverflow.com/questions/687450/how-do-i-compare-two-git-repositories
Also:
https://stackoverflow.com/questions/1968512/getting-the-difference-between-two-repositories
List all the files in a specific commit:
https://stackoverflow.com/questions/424071/how-to-list-all-the-files-in-a-commit