extends from a URL does not respect "newest = false"

[Rafael Monnerat]

The option "newest = false", has no effect when extends a URL is used, URLs are downloaded and extends are updated for every buildout run. Considering the followed buildout.cfg:

[buildout]
newest = false
extends =
  https://svn.erp5.org/repos/public/erp5/trunk/buildout/buildout-2.12.cfg
extends-cache = extends-cache

And the followed steps:

$ python bootstrap.py # All extends are downloaded into extends-cache folder.
$ bin/buildout # All extends are downloaded again into extends folder and use offline is not an option available.

The extends files should be not downloaded again during "bin/buildout" due "newest = false", and only new files should be downloaded.

I'm attaching proposal patch to fix this problem.