Staged area modified after cross branch checkout

I created two branches with commits like this:

Then I call checkout to switch the branch from old_master to master. Somehow, the cached area is updated after checkout:

Here's the code toe reproduce

import pathlib

import pygit2


def test_checkout(tmp_path: pathlib.Path):
    repo_dir = tmp_path

    git_author = pygit2.Signature(name="foo", email="foo@bar.com")
    repo = pygit2.init_repository(str(repo_dir))
    workdir = pathlib.Path(repo.workdir)
    readme = workdir / "README.md"

    readme.write_text("commit0")
    index = repo.index
    index.add(readme.relative_to(workdir).as_posix())
    index.write()
    tree = index.write_tree()
    commit0 = repo.create_commit("HEAD", git_author, git_author, "commit0", tree, [])

    master_branch = repo.branches.local["master"]
    old_master_branch = repo.branches.local.create("old_master", master_branch.peel())

    readme.write_text("commit1")
    index = repo.index
    index.add(readme.relative_to(workdir).as_posix())
    index.write()
    tree = index.write_tree()
    repo.create_commit(repo.head.name, git_author, git_author, "commit1", tree, [commit0])

    repo.checkout(old_master_branch)
    readme.write_text("commit2")
    index = repo.index
    index.add(readme.relative_to(workdir).as_posix())
    index.write()
    tree = index.write_tree()
    repo.create_commit(repo.head.name, git_author, git_author, "commit2", tree, [commit0])

    repo.checkout(master_branch)
    # the staged area should be empty after checkout
    assert not list(repo.index)

I tried it with git checkout master and it works fine without changing the staged area.

Bắt đầu bằng cách chạy trình tái hiện test_checkout được cung cấp và kiểm tra hành vi của repo.checkout và repo.index qua hai lần chuyển nhánh. So sánh index đã stage sau khi quay lại master với trạng thái trống như mong đợi, rồi xác minh kết quả dựa trên hành vi git checkout được mô tả.