[PATCH 2 of 2 stable] debian: update copyright years

[timeless]

timeless wrote:
> We should really write a test for this sort of thing. Yes it's a bit
> silly, but it'd be helpful to catch it faster than 13 months late...

Augie Fackler wrote:
> Sure, but carefully, for the reasons expressed in
> https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-February/092837.html.

Right, i'm thinking of giving us ~2 weeks tolerance and only applying
it to "releasy" files.