parse-options: set arg of abbreviated option lazily

parse-options: set arg of abbreviated option lazily

Postpone setting the opt pointer until we're about to call get_value(),
which uses it.  There's no point in setting it eagerly for every
abbreviated candidate option, which may turn out to be ambiguous.
Removing this assignment from the loop doesn't noticeably improve the
performance, but allows further simplification.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>