Otherwise we would panic:

    panic: invalid Go version: "go"

Fixes #317.

except on internal/govendor.

I also had broken the logic to always return true in a very recent
commit since the last release, for some odd reason.
Perhaps I was checking that tests for #316 were missing by
intentionally breaking the logic? Clearly I forgot that I did that.

Fixes #316.

That is, it currenty succeeds whether or not there is a diff.

This mimics the proposed and accepted behavior change for `gofmt -d`
at https://go.dev/issue/46289.

Fixes #114.