Merge branch 'ab/retire-option-argument'

An oddball OPTION_ARGUMENT feature has been removed from the
parse-options API.

* ab/retire-option-argument:
  parse-options API: remove OPTION_ARGUMENT feature
  difftool: use run_command() API in run_file_diff()
  difftool: prepare "diff" cmdline in cmd_difftool()
  difftool: prepare "struct child_process" in cmd_difftool()