parse-options: format argh like error messages

parse-options: format argh like error messages

"Keep it homogeneous across the repository" is in general a
guideline that can be used to converge to a good practice, but
we can be a bit more prescriptive in this case.  Just like the
messages we give die(_("...")) are formatted without the final
full stop and without the initial capitalization, most of the
argument help text are already formatted that way, and we want
to encourage that as the house style.

Noticed-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>