D6417: context: get filesadded() and filesremoved() from changeset if configured

[Yuya Nishihara]

I can't really comment on the storage format. I'm not keen on using extras
for this kind of stuff (including copies), but that seems be okay for
experiment.

@indygreg Any comments?

> +def decodefileindices(files, data):
> +    try:
> +        subset = []
> +        for str in data.split('\0'):
> +            i = int(str)

Better to not shadow `str()` function.

> +            if i < 0 or i > len(files):

Off by one?

> +                return None
> +            subset.append(files[i])
> +        return subset
> +    except (ValueError, IndexError):
> +        # Perhaps someone had chosen the same key name (e.g. "added") and
> +        # used different syntax for the value.