Refactored: Move CheckHeadDetach() into CAppUtils

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Warn user HEAD detached after rebase done

When choosing a non-local branch, such as FETCH_HEAD, as Branch,
the HEAD is still detached after rebase done.

Therefor, it's needed to warn user and provide a way to create new branch,
so that the changes might be not lost for further operation, such as checkout.

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>