builtin/hook.c: let parse-options parse subcommands

builtin/hook.c: let parse-options parse subcommands

'git hook' parses its currently only subcommand with an if statement.
parse-options has just learned to parse subcommands, so let's use that
facility instead, with the benefits of shorter code, handling missing
or unknown subcommands, and listing subcommands for Bash completion.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>