fix: improve self-update git pull diagnostics (#287)

Rebased and enhanced version of PR #287 by @ccqqlo:

- _run_git() now returns stderr on failure instead of empty string,
  so the UI can surface actionable git error messages
- Added _split_remote_ref() to split tracking refs like origin/master
  into separate remote + branch args for git pull
- Ignore untracked files in stash decision (--untracked-files=no) to
  prevent misleading stash-pop failures
- Fail early with clear message on unresolved merge conflicts
- 4 unit tests covering stderr, stdout fallback, exit code, and ref splitting

Based on work by @ccqqlo in PR #287.

Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_updates.py

@@ -0,0 +1,53 @@
+"""Tests for self-update diagnostics (api/updates.py)."""
+from unittest.mock import MagicMock, patch
+
+import api.updates as updates
+
+
+def test_run_git_returns_stderr_on_failure(tmp_path):
+    """When a git command fails, _run_git should return stderr (not empty string)."""
+    with patch('subprocess.run') as mock_run:
+        mock_run.return_value = MagicMock(
+            returncode=1,
+            stdout='',
+            stderr="fatal: 'origin/master' does not appear to be a git repository\n",
+        )
+        out, ok = updates._run_git(['pull', '--ff-only', 'origin/master'], tmp_path)
+
+    assert ok is False
+    assert "does not appear to be a git repository" in out
+
+
+def test_run_git_returns_stdout_when_no_stderr(tmp_path):
+    """If stderr is empty on failure, fall back to stdout."""
+    with patch('subprocess.run') as mock_run:
+        mock_run.return_value = MagicMock(
+            returncode=128,
+            stdout='Already up to date.',
+            stderr='',
+        )
+        out, ok = updates._run_git(['pull'], tmp_path)
+
+    assert ok is False
+    assert 'Already up to date' in out
+
+
+def test_run_git_returns_exit_code_when_no_output(tmp_path):
+    """If both stdout and stderr are empty, report the exit code."""
+    with patch('subprocess.run') as mock_run:
+        mock_run.return_value = MagicMock(
+            returncode=1,
+            stdout='',
+            stderr='',
+        )
+        out, ok = updates._run_git(['status'], tmp_path)
+
+    assert ok is False
+    assert 'status 1' in out
+
+
+def test_split_remote_ref_splits_tracking_ref():
+    """_split_remote_ref should correctly split origin/branch."""
+    assert updates._split_remote_ref('origin/master') == ('origin', 'master')
+    assert updates._split_remote_ref('origin/feature/foo') == ('origin', 'feature/foo')
+    assert updates._split_remote_ref('master') == (None, 'master')