[fix] Fix +=/-= support for keys in the same file

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>

src/zc/buildout/buildout.py

@@ -2308,7 +2308,10 @@ 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):