completion: adapt git-config(1) to complete subcommands
With fe3ccc7a (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 our Bash completion accordingly to advertise subcommands instead of actions.
Signed-off-by: Patrick Steinhardt ps@pks.im