git-status: do not be totally useless in a read-only repository.
git-status: do not be totally useless in a read-only repository.

This makes git-status work semi-decently in a read-only
repository.  Earlier, the command simply died with "cannot lock
the index file" before giving any useful information to the
user.

Because index won't be updated in a read-only repository,
stat-dirty paths appear in the "Changed but not updated" list.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git