commit: make it work with status.short

commit: make it work with status.short

With "status.short" set, it is now impossible to commit with
status.short set, because it acts like "git commit --short", and it
is impossible to differentiate between a status_format set by the
command-line option parser versus that set by the config parser.

To alleviate this problem, clear status_format as soon as the config
parser has finished its work.

Signed-off-by: Ramkumar Ramachandra <redacted>
Signed-off-by: Junio C Hamano <redacted>