RFC: tracking regressions with bugtags

[Robert Collins]

We have regressions from time to time. It would be nice to be able to
call them out in the bug tracker, to sensibly discuss how often and
what the impact is. And potentially to consider prioritising them
differently to other bugs, though thats a discussion with lots of
corners, and one we should have another day.

So I propose a new bug tag, and associate bug triaging rules. The tag
is 'regression', and it should be put on bugs where something has
unintentionally gotten worse than it used to be. E.g. status FILE
showing ignored versioned files twice as happens at the moment.

What say ye all?

-Rob