add--interactive: handle initial commit better

add--interactive: handle initial commit better

There were several points where we looked at the HEAD
commit; for initial commits, this is meaningless. So instead
we:

  - show staged status data as a diff against the empty tree
    instead of HEAD
  - show file diffs as creation events
  - use "git rm --cached" to revert instead of going back to
    the HEAD commit

We magically reference the empty tree to implement this.

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