Merge state v2 forward compatibility issues, and a proposal for v3

[timeless]

Siddharth Agarwal wrote:
> (5) A new advisory record type 'a' that indicates what each new mandatory
> record type 'R' does, and maybe what versions of Mercurial support it. So
> e.g. for the new change/delete conflict type 'C', ['a', 'C', 'change/delete
> conflicts (supported in Mercurial 3.7+)']

> Questions:
> (c) If so, what sort of i18n concerns would there be around (5)?

Any feature you're adding in 3.7 wouldn't be localized in 3.6, so you
couldn't rely on _(...).
You can't possibly (and wouldn't want to) serialize all languages of
your message into your file.
Roughly the best you can do is include a token which someone can search for...

I'd suggest something like: mercurial-x-y-merge-{feature} -- you could
use `_` instead of `-`, don't use spaces.