D3763: grep: add `--diff` flag

[Yuya Nishihara]

Queued, thanks.

Can you send a follow-up to deprecate the --all option?
It doesn't make sense to provide both --all and --diff in user help.

>      [('0', 'print0', None, _('end fields with NUL')),
>      ('', 'all', None, _('print all revisions that match')),
> +    ('', 'diff', None, _('print all revisions when the term was introduced or removed')),

Next time, please make sure `test-check-*` pass.