D3826: grep: change default behaviour of grep

[Yuya Nishihara]

> +    if not opts.get('rev'):
> +        opts.get('rev').append("wdir()")
> +        opts['allfiles'] = True

So, this breaks `hg grep --diff|--all`?

And this is a behavior change. We should at least document how we can get
back the original behavior.