git lfs track doesn't make file paths relative to working dir

Given a git repo in `/Users/msch/work/foo`:

```
git lfs track /Users/msch/work/foo/testcases/bigfile.pdf
```

leads to this `.gitattributes` file:

```
/Users/msch/work/foo/testcases/bigfile.pdf filter=lfs -crlf
```

when it should lead to this `.gitattributes` file:

```
testcases/bigfile.pdf filter=lfs -crlf
```
