Multi extend and += -= syntax

[Florian Schulze]

There is a bug in the += -= handling.

If you have the following cfgs.

bar.cfg

[buildout]
bar-option = bar

foo.cfg

[buildout]
foo-option = foo

buildout.cfg

[buildout]
parts =
extends = bar.cfg foo.cfg
bar-option += baz
foo-option += ham

Then the bar-option is loosing 'bar' and only contains 'baz', the foo-option is fine with 'foo\nham' (although it shouldn't really add a newline).

Annotated sections
==================

[buildout]
bar-option=
baz
+= /Users/fschulze/Zope/plone/buildouttest/buildout.cfg
bin-directory= bin
    DEFAULT_VALUE
develop-eggs-directory= develop-eggs
    DEFAULT_VALUE
directory= /Users/fschulze/Zope/plone/buildouttest
    COMPUTED_VALUE
download-cache= /Users/fschulze/Zope/buildout/downloads
    /Users/fschulze/.buildout/default.cfg
download-directory= /Users/fschulze/Zope/buildout/downloads
    /Users/fschulze/.buildout/default.cfg
eggs-directory= eggs
    DEFAULT_VALUE
executable= /Users/fschulze/Zope/python/parts/opt/bin/python2.4
    DEFAULT_VALUE
foo-option= foo
ham
    /Users/fschulze/Zope/plone/buildouttest/foo.cfg
+= /Users/fschulze/Zope/plone/buildouttest/buildout.cfg
installed= .installed.cfg
    DEFAULT_VALUE
log-format=
    DEFAULT_VALUE
log-level= INFO
    DEFAULT_VALUE
newest= false
    /Users/fschulze/.buildout/default.cfg
parts=
    /Users/fschulze/Zope/plone/buildouttest/buildout.cfg
parts-directory= parts
    DEFAULT_VALUE
python= buildout
    DEFAULT_VALUE
unzip= true
    /Users/fschulze/.buildout/default.cfg
zope-directory= /Users/fschulze/Zope/buildout/zope
    /Users/fschulze/.buildout/default.cfg