fast-import: fix read of uninitialized argv memory

fast-import: fix read of uninitialized argv memory

Fast-import shares code between its command-line parser and
the "option" command. To do so, it strips the "--" from any
command-line options and passes them to the option parser.
However, it does not confirm that the option even begins
with "--" before blindly passing "arg + 2".

It does confirm that the option starts with "-", so the only
affected case was:

  git fast-import -

which would read uninitialized memory after the argument. We
can fix it by using skip_prefix and checking the result. As
a bonus, this gets rid of some magic numbers.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>