Print output of `git describe` in `nix-prefetch-git`

The output is like this:
```
Switched to a new branch 'fetchgit'
git revision is 942100b2fe5ebf57a908171c071f7bd343106c5c
git human-readable version is v0.6.2-10-g942100b
```

I hope it will help make git-packages' version much nicer