Relative with .. doesn't work

with this:

```
|- cfgs
|  |
|  |- base.cfg
|
|- versions.cfg
|- buildout.cfg
```

where `base.cfg` has:

```
[buildout]
extends = ../versions.cfg
```

the lookup of versions.cfg fails. It should take the location of cfgs/base.cfg into account to resolve the correct path, but doesn't. I looked into the code, but couldn't figure out how to fix it. The `sub_relative` stuff looks weirdly complicated.
