status: don't require the repository to be writable

status: don't require the repository to be writable

We need to update the index before hooks run when actually making a
commit, but we shouldn't have to write the index when running "status".
If we can, then we have already spent cycles to refresh the index and
it is a waste not to write it out, but it is not a disaster if we cannot
write it out.  The main reason the user is running "git status" is to get
the "status", and refreshing the index is a mere side effect that we can
do without.

Discovery and initial attempted fix by Dscho.

Signed-off-by: Junio C Hamano <gitster@pobox.com>