fix(git): disable external diff for patch output

[stromseng]

What Changed

Appended --no-ext-diff to git diff --patch calls

Why

Having an external diff tool set in git breaks the diff display.
Closes #927

UI Changes

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Low Risk
Low risk: only adjusts git diff invocation flags to ensure consistent patch output, with no changes to data models or control flow beyond diff generation.

Overview
For all server-side git diff --patch outputs, this PR adds --no-ext-diff so configured external diff tools cannot override/garble patch text.

This affects checkpoint diff generation in CheckpointStore and patch collection in GitVcsDriverCore (staged patch for commit context and range diff patch).

^{Reviewed by Cursor Bugbot for commit 6206eea. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Disable external diff drivers for patch output in git diff calls

Adds --no-ext-diff to all git diff invocations that generate patch output, ensuring Git's internal diff is used regardless of any external diff drivers configured in the repo. Affected calls are the staged patch in prepareCommitContext, the range diff patch in readRangeContext, and checkpoint-to-checkpoint diffs in diffCheckpoints.

^{Macroscope summarized 6206eea.}

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[macroscopeapp]

Approvability

Verdict: Approved

Small defensive fix adding --no-ext-diff to git diff commands, ensuring consistent patch output regardless of user's git configuration. Clear intent, limited scope, and obvious resolution.

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[Copilot]

Pull request overview

Updates server-side Git diff invocations to ensure patch output remains in standard unified diff format even when users have configured an external diff tool (e.g., via diff.external), addressing issue #927.

Changes:

• Add --no-ext-diff to git diff --patch calls used for commit-context patch generation.
• Add --no-ext-diff to range diff patch generation for context reads.
• Add --no-ext-diff to checkpoint diff patch generation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
apps/server/src/vcs/GitVcsDriverCore.ts	Ensures unified patch output for staged and range diffs by disabling external diff tools.
apps/server/src/checkpointing/Layers/CheckpointStore.ts	Ensures checkpoint diff patch output stays parseable by disabling external diff tools.

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