CheckDirty returns an error if the current git repository is dirty.

ErrNoGit happens when git is not present in PATH.

ErrNoTag happens if the underlying git repository doesn't contain any tags but no snapshot-release was requested.

ErrNotRepository happens if you try to run goreleaser against a folder which is not a git repository.

ErrDirty happens when the repo has uncommitted/unstashed changes.

ErrWrongRef happens when the HEAD reference is different from the tag being built.

Pipe that sets up git state.