feat: rename-aware diffs with old → new origin header

[umputun]

renamed files now diff against their origin instead of rendering as fully added, and the diff pane header shows old → new.

the bug: ChangedFiles parsed git's rename pair but dropped the old path, and FileDiff ran git diff -- <newpath>. The single-path pathspec stripped the old side before git could pair the rename, so a renamed-but-edited file showed every line as added and the origin was never visible. Reproduced across --staged, single-ref, and two-ref modes.

the fix, by layer:

• FileEntry gains OldPath, populated by ChangedFiles (forced with -M so it works regardless of the user's diff.renames config)
• FileDiff takes a FileDiffRequest carrying the rename origin; (*Git).FileDiff passes -M and both paths so only the real line changes show
• the diff pane header renders old → new, sourced from a new FileTree.OldPath accessor parallel to FileStatus
• review stats and the --annotations preloader pass OldPath too, so footer counts and saved-annotation line matching agree with the displayed diff

non-git renderers ignore OldPath, so hg/jj/file/stdin/compare modes are unchanged. Rename-origin support for hg and jj is out of scope here, neither reports an origin today.

tests cover the issue's exact reproduction across all three git modes, the diff.renames=off edge, the header wiring, stats, and annotation mapping. Full suite green with -race, lint clean.

Related to #222

[Copilot]

Pull request overview

This PR fixes rename handling in Git-backed reviews so renamed-and-edited files diff against their origin (instead of rendering as a full add), and the diff pane header shows the rename origin as old → new. It does this by preserving the old path from git diff --name-status, threading it through a new diff.FileDiffRequest, and making the Git renderer request rename-aware diffs by passing both paths with -M.

Changes:

• Preserve rename origin path (OldPath) in diff.FileEntry and force rename detection in Git changed-file discovery.
• Replace positional FileDiff arguments across the codebase with diff.FileDiffRequest, and make Git diffs rename-aware when OldPath is provided.
• Wire OldPath through UI header, review stats, and annotation preloading; update docs and tests accordingly.

Reviewed changes

Copilot reviewed 44 out of 45 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
site/docs.html	Documents rename-aware behavior and Git-only scope.
README.md	Adds user-facing feature note for rename-aware diffs.
docs/plans/completed/20260606-rename-aware-diffs.md	Captures completed implementation plan and rationale for the change.
docs/ARCHITECTURE.md	Updates architecture docs for FileDiffRequest + rename-aware Git diff behavior.
CLAUDE.md	Adds repo gotcha notes for OldPath/FileDiffRequest threading and Git-only rename support.
app/ui/view.go	Renders old → new title in diff pane header when origin is known.
app/ui/view_test.go	Tests header rendering for rename vs non-rename cases.
app/ui/themeselect_test.go	Updates renderer mock signature usage to FileDiffRequest.
app/ui/sidepane/filetree.go	Stores and exposes rename origins via OldPath(path) accessor.
app/ui/sidepane/filetree_test.go	Tests FileTree.OldPath behavior and rebuild clearing/refresh.
app/ui/reviewinfo_test.go	Updates mocks/call sites to new FileDiffRequest signature.
app/ui/model.go	Updates ui.Renderer interface FileDiff signature; threads oldName via message/state.
app/ui/model_test.go	Updates renderer mocks and lookups to use FileDiffRequest.
app/ui/mocks/renderer.go	Regenerated/updated moq for new FileDiff(req) signature and call capture.
app/ui/loaders.go	Threads OldPath from file tree into diff load requests and loaded messages; staged fallback updated.
app/ui/loaders_test.go	Verifies loader threads rename origin into request/message/model.
app/ui/handlers_test.go	Updates renderer mocks to the new signature.
app/ui/diffview_test.go	Updates renderer mocks to the new signature.
app/ui/annotate_test.go	Updates renderer mocks to the new signature.
app/review/stats.go	Passes OldPath into FileDiffRequest for accurate rename-aware stats (incl. staged fallback).
app/review/stats_test.go	Adds tests ensuring OldPath is threaded and counts reflect minimal rename diffs.
app/renderer_setup_test.go	Updates renderer mocks to the new signature.
app/diff/stdin.go	Migrates stdin reader to FileDiffRequest (Path-based lookup).
app/diff/multistdin.go	Migrates multi-stdin reader to FileDiffRequest (Path-based lookup).
app/diff/multistdin_test.go	Updates tests for new FileDiffRequest signature.
app/diff/mocks/Renderer.go	Regenerated/updated moq for diff.Renderer new FileDiff(req) signature.
app/diff/jj.go	Migrates jj renderer to FileDiffRequest (uses req.Ref, req.Path, req.ContextLines).
app/diff/jj_test.go	Updates jj tests for new FileDiffRequest signature.
app/diff/include.go	Migrates include filter to pass through FileDiffRequest.
app/diff/include_test.go	Updates include filter tests for request-based signature and context threading.
app/diff/hg.go	Migrates hg renderer to FileDiffRequest.
app/diff/hg_test.go	Updates hg tests for new FileDiffRequest signature.
app/diff/hg_e2e_test.go	Updates hg/git e2e tests for new FileDiffRequest signature.
app/diff/fallback.go	Migrates fallback renderer + file reader to FileDiffRequest; uses req.Path for disk fallback.
app/diff/fallback_test.go	Updates fallback tests for new FileDiffRequest signature and context threading.
app/diff/exclude.go	Migrates exclude filter to pass through FileDiffRequest.
app/diff/exclude_test.go	Updates exclude filter tests for request-based signature and context threading.
app/diff/directory.go	Migrates directory reader to FileDiffRequest (Path-based disk read).
app/diff/directory_test.go	Updates directory reader tests for new FileDiffRequest signature.
app/diff/diff.go	Adds FileEntry.OldPath, introduces FileDiffRequest, forces -M in ChangedFiles, adds Git rename-aware pathArgs, updates old-line/binary probes.
app/diff/diff_test.go	Adds staged/refs rename reproduction tests and acceptance test for issue #222; updates existing tests to request-based API.
app/diff/compare.go	Migrates compare reader to FileDiffRequest (uses req.ContextLines).
app/diff/compare_test.go	Updates compare reader tests for new FileDiffRequest signature.
app/annotations_load.go	Threads rename origin into annotation preload diffs via renames map and FileDiffRequest.OldPath.
app/annotations_load_test.go	Adds tests ensuring rename-aware diff is used for annotation mapping; updates mocks to request-based signature.

Files not reviewed (1)

• app/diff/mocks/Renderer.go: Language not supported

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.