fix: diff shows files as deleted when run from subdirectory

[jossephus]

Running lumen diff from a subdirectory incorrectly showed modified files as deleted.

The issue was that file paths were resolved relative to the current working directory instead of the repository root. This fix adds repo_root() to VcsBackend and uses it to resolve paths correctly.

see before and after when running 'lumen diff' inside lumen/src

Summary by CodeRabbit

• Bug Fixes

  • Corrected file diff path resolution so new content is read relative to the repository root when running commands from subdirectories.

• Tests

  • Added tests verifying file diff behavior when operating from repository subdirectories.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a repo_root() accessor to the VcsBackend trait and implements it for Git and Jj backends. Updates get_new_content in the git diff command to resolve working-tree file reads relative to the repository root when available, and adds a test for reading from a subdirectory.

Changes

Cohort / File(s)	Summary
VcsBackend Trait Extension src/vcs/backend.rs	Added new repo_root(&self) -> Option<&Path> method to the VcsBackend trait.
Backend Implementations src/vcs/git.rs, src/vcs/jj.rs	Implemented repo_root() in GitBackend (returns self.repo.workdir()) and JjBackend (returns Some(&self.workspace_path)).
Path Resolution in Diff Command src/command/diff/git.rs	Modified get_new_content to join filename with backend.repo_root() when refs is WorkingTree, falling back to original path if no repo root; added test test_get_new_content_from_subdirectory.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• feat(vcs): migrate GitBackend from CLI to git2 (libgit2) #98: Modifies Git backend and diffs handling; closely related to adding repo_root() and its use in diff reading.
• feat(vcs): add VcsBackend trait and VCS detection module #88: Introduced or earlier-modified VcsBackend and backend modules that this change extends with repo_root().
• feat(vcs): implement GitBackend for git CLI integration #89: Adds repo_root() to VcsBackend and updates git usages; overlaps with this PR's trait and backend edits.

Suggested reviewers

• jnsahaj

Poem

🐰 I hop through branches, roots, and leaves,
I join your paths where the repository breathes,
From subdir shadows I fetch the new light,
No more wrong turns in the filesystem night,
Happy commits, and code that delights! 🥕

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main fix: resolving diff behavior when run from a subdirectory by using repository root paths instead of current working directory paths.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

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[Straffern]

Might be related to: #110
I think I was also encountering this, when working on my fix and I think I also solved it.

[jossephus]

oh sorry, i didnt see that. closing mine in favor of your PR since urs look complete. Thanks for letting me now