Fix test and explain purpose.

src/zc/buildout/extends-cache.txt

@@ -488,6 +488,10 @@ Unused options for buildout: 'bar' 'foo'.
 (XXX We patch download utility's API to produce readable output for the test;
 a better solution would utilise the logging already done by the utility.)
 
+As one can see, even if profile is prefixed with ../ the canocial path is
+calculated. Thanks to this paths for complex extend trees are downloaded only
+once.
+
 >>> import zc.buildout
 >>> old_download = zc.buildout.download.Download.download
 >>> def wrapper_download(self, url, md5sum=None, path=None):
@@ -502,8 +506,6 @@ 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