While refactoring the options parser in bc3c79a (fast-import: add
(non-)relative-marks feature, 2009-12-04), it was made too lenient
for options that take no argument, fix that.
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>