Preserve discarded patch context

tests/test_git.py

@@ -83,4 +83,25 @@ def test_checkpoint_restores_last_valid_changes(tmp_path) -> None:
     workspace.restore_checkpoint(checkpoint)
 
     assert tracked.read_text() == "<?php\nreturn ['valid'];\n"
-    assert added.exists()
+    assert added.exists()
+
+
+def test_diff_returns_discarded_patch_excerpt(tmp_path) -> None:
+    workspace = GitWorkspace(settings(workspace_root=tmp_path), 5)
+    workspace.path.mkdir()
+    subprocess.run(["git", "init"], cwd=workspace.path, check=True, capture_output=True)
+    tracked = workspace.path / "service.php"
+    tracked.write_text("<?php\nreturn 'old';\n")
+    subprocess.run(["git", "add", "."], cwd=workspace.path, check=True)
+    subprocess.run(
+        ["git", "-c", "user.email=test@example.invalid", "-c", "user.name=Test", "commit", "-m", "base"],
+        cwd=workspace.path,
+        check=True,
+        capture_output=True,
+    )
+    tracked.write_text("<?php\nreturn 'new';\n")
+
+    patch = workspace.diff()
+
+    assert "-return 'old';" in patch
+    assert "+return 'new';" in patch