tests/sync(test[update_repo]): Add test for rev-list HEAD failure path

why: The rev-list HEAD error handler had no test coverage, which
allowed the missing add_error() bug to go undetected.
what:
- Add test_update_repo_rev_list_head_failure_returns_sync_result

Author: tony

tests/sync/test_git.py

@@ -1025,3 +1025,37 @@ def test_update_repo_checkout_failure_returns_sync_result(
     assert result.errors[0].step == "checkout"
     assert result.errors[0].exception is not None
     assert isinstance(result.errors[0].exception, exc.CommandError)
+
+
+def test_update_repo_rev_list_head_failure_returns_sync_result(
+    create_git_remote_bare_repo: CreateRepoPytestFixtureFn,
+    tmp_path: pathlib.Path,
+) -> None:
+    """update_repo() records rev-list HEAD failure in SyncResult."""
+    git_server = create_git_remote_bare_repo()
+    git_repo = GitSync(
+        path=tmp_path / "myrepo",
+        url=git_server.as_uri(),
+    )
+    git_repo.obtain()
+
+    # Make a commit and push so the repo has a valid HEAD
+    initial_file = git_repo.path / "initial_file"
+    initial_file.write_text("content", encoding="utf-8")
+    git_repo.run(["add", str(initial_file)])
+    git_repo.run(["commit", "-m", "initial commit"])
+    git_repo.run(["push"])
+
+    # Corrupt HEAD so rev-list HEAD fails
+    head_file = git_repo.path / ".git" / "HEAD"
+    head_file.write_text("ref: refs/heads/nonexistent\n")
+
+    result = git_repo.update_repo()
+
+    assert isinstance(result, SyncResult)
+    assert result.ok is False
+    assert bool(result) is False
+    assert len(result.errors) > 0
+    assert result.errors[0].step == "rev-list-head"
+    assert result.errors[0].exception is not None
+    assert isinstance(result.errors[0].exception, exc.CommandError)