[MERGE][RFC] Add simple revision serializer based on RIO.

[Matt Nordhoff]

Martin Pool wrote:
[snip]

> However, before moving to RIO for future formats (and I say this
> having added the code) I would think hard about whether it should use
> bencode instead, which has the advantage of being able to represent
> somewhat more complex nesting (like dicts inside dicts) without
> needing a separate layer of encoding on top.  Revisions are pretty
> simple but even there it may be useful.  I'm not sure about the
> relative performance.

If I understand correctly, RIO is line-based but bencode is not. Is the
delta format still line-based? If so, using bencode would be more difficult.
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