wt-status: refactor initial commit printing

wt-status: refactor initial commit printing

When we showed the initial commit, we had no reference to
diff against, so we went through the cache manually.
Nowadays, however, we have a virtual empty tree commit, so
we can simply diff against that to get the same results.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>