fix(git): pass --no-ext-diff when diffing untracked files

[iamken1204]

Summary

If you have diff.external set in git config (difftastic, delta, etc.), hunk diff crashes on any untracked file:

$ hunk diff
hunk: Expected one parsed file for untracked patch "foo.txt", got 0.

hunk forms untracked patches via git diff --no-index, which honors diff.external and outputs whatever that tool prints instead of unified diff. The regular diff/show paths already use --no-ext-diff, the untracked path was missing it.

Test

• Added a regression that sets diff.external on a temp repo and loads an untracked file.
• Repro'd locally with diff.external = difft; works after the fix.

[greptile-apps]

Greptile Summary

This PR fixes a crash in hunk diff when a user has diff.external configured in git (e.g. difftastic, delta): the untracked-file synthetic-patch path was missing --no-ext-diff, so git replaced unified-diff output with whatever the external tool produces, which the parser couldn't handle. A regression test covering the exact failure mode is also added.

• src/core/git.ts — adds --no-ext-diff to buildGitNewFileDiffArgs, aligning it with buildGitDiffArgs and buildGitShowArgs which already carry the flag. buildGitStashShowArgs is still missing it (see inline comment).
• src/core/loaders.test.ts — adds a regression test that writes diff.external=/usr/bin/true into a temp repo config and asserts an untracked file loads without error.

Confidence Score: 3/5

The untracked-file fix is correct and well-tested, but hunk stash show carries the same unfixed bug for users with diff.external configured.

The fix is correct and directly addresses the reported crash. However, buildGitStashShowArgs is still missing --no-ext-diff — the same condition that broke the untracked path will produce an identical parse failure when a user runs hunk stash show with diff.external set. The PR partially solves the problem it sets out to fix.

src/core/git.ts — specifically buildGitStashShowArgs which should mirror the flag added to the other three diff-producing functions.

Important Files Changed

Filename	Overview
src/core/git.ts	Adds --no-ext-diff to buildGitNewFileDiffArgs fixing the untracked-file crash; buildGitStashShowArgs is still missing the same flag and will break with diff.external configured.
src/core/loaders.test.ts	Adds a targeted regression test that configures diff.external=/usr/bin/true in a temp repo and asserts the untracked file is loaded correctly; well-scoped and sufficient for the fix it covers.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[hunk diff] --> B{File type?}
    B -->|Tracked changes| C[buildGitDiffArgs\n--no-ext-diff ✓]
    B -->|Commit show| D[buildGitShowArgs\n--no-ext-diff ✓]
    B -->|Untracked file| E[buildGitNewFileDiffArgs\n--no-ext-diff ✓ FIXED]
    B -->|Stash show| F[buildGitStashShowArgs\n--no-ext-diff ✗ MISSING]
    C --> G[Unified diff output → parser]
    D --> G
    E --> G
    F -->|diff.external configured| H[External tool output → parser CRASH]
    F -->|No diff.external| G

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Comments Outside Diff (1)

1. src/core/git.ts, line 104 (link)

   buildGitStashShowArgs is missing --no-ext-diff, the same flag this PR adds to buildGitNewFileDiffArgs. A user with diff.external set will get the same unparseable output from hunk stash show that motivated this fix — git stash show -p honors diff.external just like git diff --no-index does.

   Prompt To Fix With AI

   This is a comment left during a code review.
   Path: src/core/git.ts
   Line: 104
   
   Comment:
   `buildGitStashShowArgs` is missing `--no-ext-diff`, the same flag this PR adds to `buildGitNewFileDiffArgs`. A user with `diff.external` set will get the same unparseable output from `hunk stash show` that motivated this fix — `git stash show -p` honors `diff.external` just like `git diff --no-index` does.
   
   
   
   How can I resolve this? If you propose a fix, please make it concise.

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
src/core/git.ts:104
`buildGitStashShowArgs` is missing `--no-ext-diff`, the same flag this PR adds to `buildGitNewFileDiffArgs`. A user with `diff.external` set will get the same unparseable output from `hunk stash show` that motivated this fix — `git stash show -p` honors `diff.external` just like `git diff --no-index` does.

```suggestion
  const args = ["stash", "show", "-p", "--no-ext-diff", "--find-renames", "--no-color"];
```

_{Reviews (1): Last reviewed commit: "fix(git): pass --no-ext-diff when synthe..." | Re-trigger Greptile}

[benvinegar]

Addressed the Greptile note by making hunk stash show pass --no-ext-diff too, so all Git-backed patch-producing paths now explicitly avoid external diff tools. I also adjusted the existing untracked-file regression to avoid the Unix-only /usr/bin/true command.

This comment was generated by Pi using OpenAI GPT-5