Support "binary" language override, meaning don't diff

[gthb]

Add a binary (or none or no-diff) “language”, which simply means “skip files of this type”, similar to Git's -diff attribute in .gitattributes.

Motivation: my repo contains some test fixtures compressed with brotli, named *.json.br. Git sometimes mistakes them for text, so diffs get useless noise in them — so I put this in fixtures/.gitattributes:

*.br -diff

and that works, git diff (without Difftastic) just says “Binary files differ”.

But Difftastic, via git -c diff.external=difft, still shows the diff, presumably because .gitattributes is not consulted (because it's an external diff).

If Difftastic defined a “binary” language, then I could just use -c external.diff='difft --override="*.br:binary" to tell it what I want.

(Or is there a simple way to achieve this with Difftastic as-is?)

[gthb]

Alternatively, in Git I can set up a diff driver with textconv to decompress files for diffing, using this in .gitattributes:

*.br diff=brotli

and this in .gitconfig:

[diff "brotli"]
    textconv = brotli -cd

and then git diff acts on the textconv result so I get diffs despite the compression.

But this also doesn't work with Difftastic configured via external.diff, because Git's external diff option bypasses the diffing machinery, including textconv filters. I imagine it's a bit of a lift to add this to Difftastic directly.

[Wilfred]

Sooner or later I'll need to add support for .gitattributes I think, it's been mentioned a couple of times in other issues.

Having a way for --override to force "binary" does seem reasonable too. Note that difftastic 0.64 is stricter about text encoding, so that might also help it treat brotli compressed files as binary.