fix: neutralize git config that alters diff format

packages/git/src/diff.ts

@@ -1,7 +1,22 @@
 import { exec, execLarge, execLines, execWithStdin } from './exec.js';
 
+/**
+ * Flags that neutralize user git config which would otherwise alter the diff
+ * format and break parsing:
+ * - `--no-color` guards against `color.ui=always` / `color.diff=always`
+ * - `--no-ext-diff` guards against a configured `diff.external` driver
+ * - `--src-prefix`/`--dst-prefix` force the standard `a/`/`b/` prefixes,
+ *   overriding `diff.mnemonicPrefix`, `diff.noprefix` and custom prefixes
+ */
+const DIFF_FORMAT_ARGS = [
+  '--no-color',
+  '--no-ext-diff',
+  '--src-prefix=a/',
+  '--dst-prefix=b/',
+];
+
 export function getDiff(args: string[] = []): string {
-  const cmd = ['git', 'diff', ...args].join(' ');
+  const cmd = ['git', 'diff', ...DIFF_FORMAT_ARGS, ...args].join(' ');
   return execLarge(cmd);
 }
 
@@ -14,7 +29,7 @@ export function getUntrackedDiff(files: string[]): string {
 
   for (const file of files) {
     try {
-      execLarge(`git diff --no-index -- /dev/null "${file}"`);
+      execLarge(`git diff ${DIFF_FORMAT_ARGS.join(' ')} --no-index -- /dev/null "${file}"`);
     } catch (err: unknown) {
       const error = err as { stdout?: string; status?: number };
       if (error.status === 1 && error.stdout) {

packages/git/tests/diff-format.test.ts

@@ -0,0 +1,70 @@
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { execSync } from 'node:child_process';
+import { mkdtempSync, writeFileSync, rmSync } from 'node:fs';
+import { join } from 'node:path';
+import { tmpdir } from 'node:os';
+
+let repoDir: string;
+let origCwd: string;
+
+function git(cmd: string) {
+  execSync(`git ${cmd}`, { cwd: repoDir, stdio: 'pipe' });
+}
+
+function writeFile(name: string, content: string) {
+  writeFileSync(join(repoDir, name), content);
+}
+
+beforeAll(() => {
+  origCwd = process.cwd();
+  repoDir = mkdtempSync(join(tmpdir(), 'diffity-format-test-'));
+
+  git('init -b main');
+  git('config user.email "test@test.com"');
+  git('config user.name "Test"');
+
+  // git config that alters the default diff format and previously broke parsing
+  git('config diff.mnemonicprefix true');
+  git('config color.ui always');
+
+  writeFile('base.txt', 'a\nb\n');
+  git('add .');
+  git('commit -m "initial commit"');
+
+  process.chdir(repoDir);
+});
+
+afterAll(() => {
+  process.chdir(origCwd);
+  rmSync(repoDir, { recursive: true, force: true });
+});
+
+describe('diff format neutralization', () => {
+  it('emits standard a/ b/ prefixes despite diff.mnemonicprefix', async () => {
+    const { resolveRef } = await import('../src/diff.js');
+    writeFile('base.txt', 'a\nc\n');
+
+    const raw = resolveRef('unstaged');
+
+    expect(raw).toContain('diff --git a/base.txt b/base.txt');
+    expect(raw).toContain('--- a/base.txt');
+    expect(raw).toContain('+++ b/base.txt');
+    // no ANSI color escapes even with color.ui=always
+    expect(raw).not.toMatch(/\x1b\[/);
+
+    git('checkout -- base.txt');
+  });
+
+  it('emits standard prefixes for untracked files', async () => {
+    const { resolveRef } = await import('../src/diff.js');
+    writeFile('untracked.txt', 'x\ny\n');
+
+    const raw = resolveRef('work');
+
+    expect(raw).toContain('diff --git a/untracked.txt b/untracked.txt');
+    expect(raw).toContain('+++ b/untracked.txt');
+    expect(raw).not.toMatch(/\x1b\[/);
+
+    execSync(`rm "${join(repoDir, 'untracked.txt')}"`, { stdio: 'pipe' });
+  });
+});