difftool -d: ensure that intent-to-add files are handled correctly

difftool -d: ensure that intent-to-add files are handled correctly

In https://github.com/git-for-windows/git/issues/2677, a `git difftool
-d` problem was reported. The underlying cause was a bug in `git
diff-files --raw` that we just fixed: it reported intent-to-add files
with the empty _tree_ as the post-image OID, when we need to show
an all-zero (or, "null") OID instead, to indicate to the caller that
they have to look at the worktree file.

The symptom of that problem shown by `git difftool` was this:

	error: unable to read sha1 file of <path> (<empty-tree-OID>)
	error: could not write '<filename>'

Make sure that the reported `difftool` problem stays fixed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>