Feature idea: optional structural / external diff view

[rs545837]

Hi — huge fan of gitui, it's one of my daily drivers.

A thought: gitui shows line-level diffs from libgit2. For some changes a structural view (which functions/types changed, not which lines) is easier to scan, especially in big refactors. Would you be open to an optional "external/structural diff" view — a configurable command gitui pipes the diff through and renders, similar to how the external editor works? That'd let people plug in difftastic, delta, or sem (a semantic, entity-level diff I work on) without gitui taking on any specific dependency.

Happy to prototype it as a PR if you're open to the idea — wanted to check direction first.

[extrawurst]

This is mostly discussed over in #358