[Engine] Move `git_repo.dart` to `pkg/git_repo_tools`

I'd like to re-use the tooling for https://github.com/flutter/flutter/issues/133415#issuecomment-1724587433.

/cc @zanderso, this should be a very small change, but just wanted to FYI you.