fault: ignore when lines are moved within or between files

On [teachers-pet](https://github.com/education/teachers-pet):

```
# BEFORE
~/dev/teachers-pet $ git fault .
 926 Aidan Feldman
  56 Mike Helmick
  25 John Britton
   4 kelleydv
# AFTER
~/dev/teachers-pet $ git fault .
 499 Aidan Feldman
 463 Mike Helmick
  31 John Britton
  14 Matthew Hartman
   4 kelleydv
```

Much more accurate :+1: 

http://git-scm.com/docs/git-blame

/cc @mikehelmick