fixup! Write .installed.cfg only once, in safe way and only if there's any change.

Because updating a section no longer add the section t the end of parts in
.installed, uninstallation happens in this order.

src/zc/buildout/buildout.txt

@@ -2660,9 +2660,9 @@ provide alternate locations, and even names for these directories::
     Creating directory '/sample-alt/work'.
     Creating directory '/sample-alt/developbasket'.
     Develop: '/sample-buildout/recipes'
-    Uninstalling d4.
     Uninstalling d2.
     Uninstalling debug.
+    Uninstalling d4.
     Uninstalling d3.
 
     >>> ls(alt)