parse-options: fully disable option abbreviation with PARSE_OPT_KEEP_UNKNOWN

parse-options: fully disable option abbreviation with PARSE_OPT_KEEP_UNKNOWN

baa4adc66a (parse-options: disable option abbreviation with
PARSE_OPT_KEEP_UNKNOWN, 2019-01-27) turned off support for abbreviated
options when the flag PARSE_OPT_KEEP_UNKNOWN is given, as any shortened
option could also be an abbreviation for one of the unknown options.

The code for handling abbreviated options is guarded by an if, but it
can also be reached via goto.  baa4adc66a only blocked the first way.
Add the condition to the other ones as well.

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