completion: adapt git-config(1) to complete subcommands

completion: adapt git-config(1) to complete subcommands

With fe3ccc7aab (Merge branch 'ps/config-subcommands', 2024-05-15),
git-config(1) has gained support for subcommands. These subcommands live
next to the old, action-based mode, so that both the old and new way
continue to work.

The manpage for this command has been updated to prominently show the
subcommands, and the action-based modes are marked as deprecated. Update
Bash completion scripts accordingly to advertise subcommands instead of
actions.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>