builtin/commit.c: set status_format _after_ option parsing

builtin/commit.c: set status_format _after_ option parsing

'git status' should use --porcelain output format when -z is given.
It was not doing so since the _effect_ of using -z, namely that
null_termination would be set, was being checked _before_ option parsing
was performed.

So, move the check so that it is performed after option parsing.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>