Add native difftastic usage and remove git diff fallback

[martinabeleda]

This PR replaces ripdiff's external difft subprocess integration with an in-process Rust integration built on a forked difftastic dependency.

Instead of invoking git diff with GIT_EXTERNAL_DIFF=difft, ripdiff now prepares the relevant git/worktree snapshots in Rust and renders the diff through difftastic directly in-process.

My fork of difftastic was modified to a library in martinabeleda/difftastic#1

Related upstream discussion: Wilfred/difftastic#216

Changes

• add difftastic as a Git dependency in Cargo.toml
• replace the subprocess-based diff path in src/diff.rs
• remove the external difft runtime requirement
• remove the old plain git diff fallback

Notes

This uses a forked difftastic repo because upstream does not currently expose a supported library crate.

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