Before this, += and -= worked correctly when applied to a key defined
in a file extended by the current file, but not when the key was in
the same file.

e.g.:

```
[part]
a = this case is
```

```
[buildout]
extends = a.cfg

[part]
a += ok
b = this case is
b += not-ok
```
Co-authored-by: Xavier Thompson <xavier.thompson@nexedi.com>