feat: syntax aware diff

[zintus]

Is it worthy exploring https://github.com/Wilfred/difftastic style differ to get higher SNR diffs?

They seem to provide machine readable output format

difft --display=json old.go new.go

[umputun]

looked at difftastic before, the model is interesting but doesn't fit revdiff's design.

revdiff is built around unified-diff line format end to end. Renderer.FileDiff returns []DiffLine tagged Add/Remove/Context/Divider, annotations are addressed by line number, and search/hunk navigation/scroll all key off that line model. Difftastic's output is structural, AST-level moves and reorderings that don't map cleanly to +/- per line. Its JSON exists, but projecting it back onto a synthetic line stream loses most of the SNR benefit you'd want in the first place.

a few other practical issues:

• tree-sitter is per-language grammars, chroma covers everything via lexers. Swapping to difftastic's grammar set drops support for long-tail languages we currently highlight fine.
• adds an external binary dependency. Today revdiff only needs git/hg/jj.
• intra-line word-diff already covers the most common case (renamed identifiers, small edits in long lines) via LCS in app/ui/worddiff/. That's the part where higher SNR actually shows up in practice for me.

so not planning to pursue this. If you have a concrete scenario where the current diff (with word-diff on) fails badly, drop a sample, that's more useful than the general direction.