fix(git): don't use external diff tools when getting git diff

[muni-corn]

i use difftastic, and it was common for LLMs to think that no diff was provided because difftastic can eliminate enough context to make diffs person-readable, but not LLM-readable.

adding --no-ext-diff to our git diff command should help to avoid this.

[muni-corn]

@di-sukharev Any thoughts on this? 😁

[di-sukharev]

Adding --no-ext-diff is correct and still needed because the current master does not include it. This PR conflicts after the getDiff refactor, so please port the single change onto master and add a regression test with an external diff tool configured, verifying that OpenCommit still receives the regular staged patch.

[muni-corn]

done! regression test pending

[di-sukharev]

The implementation is correct and now rebased cleanly onto master. I added the requested end-to-end regression coverage for a configured external diff tool and regenerated the bundled CLI. Locally verified lint/typecheck, 170 unit tests, and all 34 core E2E tests. Ready to merge.

[di-sukharev]

All done — I added end-to-end regression coverage for repositories with a configured external diff tool, regenerated the bundled CLI, and merged the PR into master. Thank you for the contribution!