diff*: fix worktree setup

diff*: fix worktree setup

This fixes "git diff", "git diff-files" and "git diff-index" to work
correctly under worktree setup. Because diff* family works in many modes
and not all of them require worktree, Junio made a nice summary
(with a little modification from me):

 * diff-files is about comparing with work tree, so it obviously needs a
  work tree;

 * diff-index also does, except "diff-index --cached" or "diff --cached TREE"

 * no-index is about random files outside git context, so it obviously
   doesn't need any work tree;

 * comparing two (or more) trees doesn't;

 * comparing two blobs doesn't;

 * comparing a blob with a random file doesn't;

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>