builtin/config: display subcommand help

builtin/config: display subcommand help

Until now, `git config -h` would have printed help for the old-style
syntax. Now that all modes have proper subcommands though it is
preferable to instead display the subcommand help.

Drop the `NO_INTERNAL_HELP` flag to do so. While at it, drop the help
mismatch in t0450 and add the `--get-colorbool` option to the usage such
that git-config(1)'s synopsis and `git config -h` match.

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