Make extensions deeper.

src/zc/buildout/extends-cache.txt

@@ -466,6 +466,7 @@ used:
 >>> write(server_data, 'baseD.cfg', """\
 ... [buildout]
 ... extends-cache = cache
+... extends = baseC.cfg
 ... bar = foo
 ... """)
 >>> mkdir(server_data, 'deeper')
@@ -501,6 +502,8 @@ The URL http://localhost/baseB.cfg was downloaded.
 The URL http://localhost/baseC.cfg was downloaded.
 The URL http://localhost/deeper/base.cfg was downloaded.
 The URL http://localhost/baseD.cfg was downloaded.
+The URL http://localhost/baseC.cfg was downloaded.
+The URL http://localhost/baseB.cfg was downloaded.
 Unused options for buildout: 'bar' 'foo'.
 
 >>> zc.buildout.download.Download.download = old_download