"git diff": do not ignore index without --no-index

Even if "foo" and/or "bar" does not exist in index, "git diff foo bar"
should not change behaviour drastically from "git diff foo bar baz" or
"git diff foo".  A feature that "sometimes works and is handy" is an
unreliable cute hack.

"git diff foo bar" outside a git repository continues to work as a more
colourful alternative to "diff -u" as before.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4013/diff.diff_--name-status_dir2_dir

@@ -1,3 +1,2 @@
 $ git diff --name-status dir2 dir
-A	dir/sub
 $