status: only touch path we may need to check

status: only touch path we may need to check

This patch gets rid of whole-tree cache refresh and untracked file
search. Instead only specified path will be looked at.

Again some numbers on gentoo-x86, ~80k files:

Unmodified Git:

$ time git st eclass/
nothing to commit (working directory clean)

real    0m3.211s
user    0m1.977s
sys     0m1.135s

Modified Git:

$ time ~/w/git/git st eclass/
nothing to commit (working directory clean)

real    0m1.587s
user    0m1.426s
sys     0m0.111s

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