Make "-h" command line option work more consistently in all commands.
* rs/parseopt-short-help:
show-ref: stop using PARSE_OPT_NO_INTERNAL_HELP
grep: stop using PARSE_OPT_NO_INTERNAL_HELP
parse-options: allow -h as a short option
parse-options: inline parse_options_usage() at its only remaining caller
parse-options: deduplicate parse_options_usage() calls