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>