builtin/config: move legacy mode into its own function

builtin/config: move legacy mode into its own function

In `cmd_config()` we first try to parse the provided arguments as
subcommands and, if this is successful, call the respective functions
of that subcommand. Otherwise we continue with the "legacy" mode that
uses implicit actions and/or flags.

Disentangle this by moving the legacy mode into its own function. This
allows us to move the options into the respective functions and clearly
separates concerns.

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