Merge branch 'jk/unused-post-2.40-part2'

Merge branch 'jk/unused-post-2.40-part2'

Code clean-up for "-Wunused-parameter" build.

* jk/unused-post-2.40-part2:
  parse-options: drop parse_opt_unknown_cb()
  t/helper: mark unused argv/argc arguments
  mark "argv" as unused when we check argc
  builtins: mark unused prefix parameters
  builtins: annotate always-empty prefix parameters
  builtins: always pass prefix to parse_options()
  fast-import: fix file access when run from subdir