[MERGE] Fix #296620 by allowing plugins instead of requiring them.

[Andrew Bennetts]

Aaron Bentley wrote:
[...]
> There are other ways we could accomplish this.  A registry of
> diff_writers is the most obvious approach that comes to mind.

I think this is what we should do.  This is how plugins can modify/extend
the behaviour elsewhere in bzrlib, and it works well.  For both consistency
and fixing this bug it seems like the right way to handle this feature too.

-Andrew.