diff --no-index: allow pathspec after --

Just checking if we're ok with this direction. I can split/cleanup
this patch later.

Commit d516c2d (Teach git-diff-files the new option `--no-index` -
2007-02-22) brings the bells and whistles of git-diff to the world
outside a git repository. This patch continues that direction and adds
a new syntax

    git diff --no-index [--] <path> <path> -- <pathspec>

It's easy to guess that the two directories will be filtered by
pathspec, which could be useful for filtering out generated files
(negative pathspec to rescue!), or simply to limit diff output to a
selection of files.