Apply += & -= in correct order with respect to extends

src/zc/buildout/buildout.py

@@ -1985,10 +1985,8 @@ def _update_section(s1, s2):
 
 def _update(d1, d2):
     for section in d2:
-        if section in d1:
-            d1[section] = _update_section(d1[section], d2[section])
-        else:
-            d1[section] = d2[section]
+        # always call _update_section, so that += & -= are processed correctly
+        _update_section(d1.setdefault(section, {}), d2[section])
     return d1
 
 def _recipe(options):