Merge branch 'ab/commit-graph-usage'

Fixes on usage message from "git commit-graph".

* ab/commit-graph-usage:
  commit-graph: show "unexpected subcommand" error
  commit-graph: show usage on "commit-graph [write|verify] garbage"
  commit-graph: early exit to "usage" on !argc
  multi-pack-index: refactor "goto usage" pattern
  commit-graph: use parse_options_concat()
  commit-graph: remove redundant handling of -h
  commit-graph: define common usage with a macro