[tmp]: Fix += and -= in the same file

src/zc/buildout/buildout.py

@@ -2127,8 +2127,6 @@ class _extends(object):
         result = {}
         for d in self.extends:
           _update(result, d)
-          # Hack: Update with itself to process += in the same file
-          _update(result, result)
         return result
 
     def __getattr__(self, attr):
@@ -2313,7 +2311,10 @@ def _update(d1, d2):
         if section in d1:
             _update_section(d1[section], d2[section])
         else:
-            d1[section] = d2[section]
+            # In order to process += (and -=) correctly when
+            # <key> = <value> and <key> += <value> are in the same section
+            # _update_section must be called even when section is not in d1
+            d1[section] = _update_section({}, d2[section])
     return d1
 
 def _recipe(options):