'cabal get': don't add the version number to the dir name when forking.

Fixes #1279.

Test:

```
$ cabal get binary
Downloading binary-0.7.0.1...
Unpacking to binary-0.7.0.1/

$ cabal get binary -s
git: clone "git://github.com/kolmodin/binary.git"
Cloning into 'binary'...
remote: Counting objects: 2912, done.
remote: Compressing objects: 100% (1383/1383), done.
remote: Total 2912 (delta 1184), reused 2868 (delta 1145)
Receiving objects: 100% (2912/2912), 571.62 KiB | 460 KiB/s, done.
Resolving deltas: 100% (1184/1184), done.
```