commit: make it work with status.short

50e4f75 (status: introduce status.short to enable --short by default,
2013-06-11) introduced a regression in git commit; it is now impossible
to commit with status.short set.

This happens because commit internally runs run_status() to set
s->commitable and determine whether or not there is something to
commit.  The problem arises from the fact that only STATUS_FORMAT_NONE
(or STATUS_FORMAT_LONG) is equipped to set s->commitable.
7c9f7038 (commit: support alternate status formats, 2009-09-05) clearly
states that --short and --porcelain imply --dry-run and are therefore
only intended for display purposes.

The bigger problem is that it is impossible to differentiate between a
status_format set by the command-line option parser versus that set by
the config parser.  So these two are exactly equivalent:

  $ git -c status.short=true commit
  $ git commit --short

To alleviate this problem, clear status_format as soon as the config
parser has finished its work to nullify the effect of parsing
status.short, and get commit to work again.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>