If some of the branches have no tags at all (like in the case
of feature branches or early master branches), the listing of
branches in list-changes fails.
This problem was introduced in I4f06d6cfaa8ea332d9326ccd8a5e69c749589529
since we switched to subprocess' check_output instead of call.
Change-Id: Id8c61662ecf3a9424c3b4e21fb36f0ed8c01d6b1