debug abi breaking change patch

[Jonathan Wakely]

>          * include/debug/safe_base.h (_Safe_iterator_base::_M_get_mutex):
>          Remove. (_Safe_sequence_base::_M_get_mutex): Remove and replace
>          with _M_mutex.

The changelog format is wrong, the function names in parentheses
should be on a new line.

But otherwise, I think (someone please correct me if I'm wrong) you
can commit to your own branches without approval. This change looks
good to me anyway.