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