After upgrading bash-completion to 2.12, grub2 bash-completion scripts stopped
to work and showed the error: _split_longopt: command not found.

It's because bash-completion 2.12 renamed '_split_longopt' as
'_comp__split_longopt', and bash-completion upstream(*) stated that
'_split_longopt' is a private API so it should not be used directly. Instead,
the script should use '_init_completion -s' or the newer '_comp_initialize -s'
to initialize 'cur', 'prev', and 'split'.

(*) https://github.com/scop/bash-completion/issues/1135