tl;dr: Copybara is a Google-internal tool, available on GitHub, for transforming and syncing code between repositories—commonly used to keep a confidential repo in sync with a public one, or to import external contributions back into an authoritative source. It's stateless (storing state as labels in destination commit messages), configured via Starlark-like files (copy.bara.sky), and currently supports Git with experimental Mercurial read support. Workflows define origin, destination, file globs, and transformations like path moves and string replacements.