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()