Add path-limiting to git-add--interactive

Add path-limiting to git-add--interactive

Implement Junio's suggestion that git-add--interactive should reproduce the
path-limiting semantics of non-interactive git-add.

In otherwords, if "git add -i" (unrestricted) shows paths from a set A,
"git add -i paths..." should show paths from a subset of the set A and that
subset should be defined with the existing ls-files pathspec semantics.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>