maintenance: add parse-options boilerplate for subcommands

maintenance: add parse-options boilerplate for subcommands

Several of the git-maintenance subcommands don't take any options, so
they don't bother looking at argv at all. This means they'll silently
accept garbage, like:

  $ git maintenance register --foo
  [no output]

  $ git maintenance stop bar
  [no output]

Let's give them the basic boilerplate to detect and handle these cases:

  $ git maintenance register --foo
  error: unknown option `foo'
  usage: git maintenance register

  $ git maintenance stop bar
  usage: git maintenance stop

We could reduce the number of lines of code here a bit with a shared
helper function. But it's worth building out the boilerplate, as it may
serve as the base for adding options later.

Note one complication: maintenance_start() calls directly into
maintenance_register(), so it now needs to pass a plausible argv (we
don't care, but parse_options() is expecting there to at least be an
argv[0] program name). This is an extra line of code, but it eliminates
the need for an explanatory comment.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>