The sg/parse-options-subcommand topic was merged to 'next', and it
introduced a ton of -Wunused-parameter warnings. But I think in this
case it revealed some actual bugs, albeit minor.

Here are some fixes. I prepared them on top of that topic, which is
semantically necessary. But note that the topic is not itself at fault
for _introducing_ any bugs. It just revealed them. So I don't think
there's any need to pause its progress to master.

  [1/3]: pass subcommand "prefix" arguments to parse_options()
  [2/3]: maintenance: add parse-options boilerplate for subcommands
  [3/3]: remote: run "remote rm" argv through parse_options()

 builtin/commit-graph.c     |  4 ++--
 builtin/gc.c               | 43 +++++++++++++++++++++++++++++++++++++-
 builtin/multi-pack-index.c |  8 +++----
 builtin/remote.c           | 36 ++++++++++++++++++-------------
 builtin/sparse-checkout.c  |  8 +++----
 5 files changed, 73 insertions(+), 26 deletions(-)

-Peff