Don't include files not tracked by git when looking for uncommitted files.

[oalders]

This can provide a big speed boost in situations where checking for untracked files can be quite slow.

vagrant@ct100-test:~/mm_website$ time git status --porcelain -z
?? .DS_Store?? doc/.DS_Store
real    0m1.628s
user    0m0.080s
sys     0m0.592s
vagrant@ct100-test:~/mm_website$ time git status --porcelain -z -uno

real    0m0.077s
user    0m0.015s
sys     0m0.061s