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

In #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 <[email protected]>

Author: dscho

t/t7800-difftool.sh

@@ -720,6 +720,14 @@ test_expect_success SYMLINKS 'difftool --dir-diff handles modified symlinks' '
 	test_cmp expect actual
 '
 
+test_expect_success 'add -N and difftool -d' '
+	test_when_finished git reset --hard &&
+
+	test_write_lines A B C >intent-to-add &&
+	git add -N intent-to-add &&
+	git difftool --dir-diff --extcmd ls
+'
+
 test_expect_success 'outside worktree' '
 	echo 1 >1 &&
 	echo 2 >2 &&