D3553: notify: add option to include function names in the diff output

[Yuya Nishihara]

> +  notify.show-functions
> +    Set to True to get the equivalent of "hg diff --show-functions". Default:
> +    False

So is this equivalent to setting the diff option?

```
[diff]
showfunc = True
```

I'm not sure if we need `notify.show-functions` in addition to `diff.*`
options, but if we do need it, it's probably better to copy the option
name from `[diff]`.