fix

tests/test_diff.py

@@ -38,8 +38,11 @@ def test_run_git_diff_falls_back_to_origin_refs_after_fetch():
     def fake_run(cmd, check=False, capture_output=False, text=False):  # type: ignore[override]
         nonlocal fetched
         if cmd[3] == "diff":
-            diff_attempts.append(cmd[4])
-            spec = cmd[4]
+            if len(cmd) == 7:
+                spec = cmd[4]
+            else:
+                spec = " ".join(cmd[4:6])
+            diff_attempts.append(spec)
             if spec == "origin/base...origin/head" and fetched:
                 return _completed(cmd, 0, stdout="remote-ok")
             return _completed(cmd, 128, stderr="missing ref")
@@ -55,3 +58,29 @@ def test_run_git_diff_falls_back_to_origin_refs_after_fetch():
     assert "origin/base...origin/head" in diff_attempts
     # Ensure the fallback only succeeds after fetch.
     assert diff_attempts.index("origin/base...origin/head") > diff_attempts.index("base...head")
+
+
+def test_run_git_diff_fallbacks_to_two_dot_when_merge_base_missing():
+    fetched = False
+    diff_attempts: list[str] = []
+
+    def fake_run(cmd, check=False, capture_output=False, text=False):  # type: ignore[override]
+        nonlocal fetched
+        if cmd[3] == "diff":
+            if len(cmd) == 7:
+                spec = cmd[4]
+                diff_attempts.append(f"sym:{spec}")
+                return _completed(cmd, 128, stderr="fatal: origin/main...origin/head: no merge base")
+            spec = " ".join(cmd[4:6])
+            diff_attempts.append(f"two:{spec}")
+            return _completed(cmd, 0, stdout="linear-diff")
+        if cmd[3] == "fetch":
+            fetched = True
+            return _completed(cmd, 0)
+        raise AssertionError("unexpected git invocation")
+
+    with patch("ai_reviewer.diff.subprocess.run", side_effect=fake_run):
+        output = run_git_diff(".", "base", "head")
+
+    assert output == "linear-diff"
+    assert any(attempt.startswith("two:") for attempt in diff_attempts)