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

If a.cfg extends b.cfg and both contain [a],
it was already not possible for a.cfg:[a] to combine <= & +=.
But it worked when a.cfg:[a] does not extend anything, and
commit 161fb191 broke that.

The only proper way to solve all this seems to process <= before
+= & -=. But since processing <= while extending files would cause
other issues, we'd rather process +=&-= when initializing parts.

src/zc/buildout/buildout.py

@@ -1985,9 +1985,16 @@ def _update_section(s1, s2):
     return s1
 
 def _update(d1, d2):
-    for section in d2:
-        # always call _update_section, so that += & -= are processed correctly
-        _update_section(d1.setdefault(section, {}), d2[section])
+    for section, d2 in d2.items():
+        if section in d1:
+            _update_section(d1[section], d2)
+        else:
+            # XXX: In order to process to process += & -= correctly
+            #      with respect to extends, we want to use _update_section
+            #      even if d1 does not contain the section.
+            #      However, this would break the only case where <= can be
+            #      combined with += & -=.
+            d1[section] = d2 if '<' in d2 else _update_section({}, d2)
     return d1
 
 def _recipe(options):