Merge branch 'rs/parse-options-help-text-is-optional'

It may be tempting to leave the help text NULL for a command line option that is either hidden or too obvious, but "git subcmd -h" and "git subcmd --help-all" would have segfaulted if done so. Now the help text is optional. * rs/parse-options-help-text-is-optional: parse-options: allow omitting option help text