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

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

'git notes' parses its subcommands with a long list of if-else if
statements.  parse-options has just learned to parse subcommands, so
let's use that facility instead, with the benefits of shorter code,
handling unknown subcommands, and listing subcommands for Bash
completion.  Make sure that the default operation mode doesn't accept
any arguments.

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