Comment 3 for bug 717802

Included a fix and test for when a relative url is used in extends.

Example:

-- buildout.cfg content --
[buildout]
extends =
  http://url/to/file.cfg
  http://url/to/file2.cfg
  http://url/to/file3.cfg

-- http://url/to/file.cfg content --
[buildout]
extends =
  http://url/to/sample1.cfg

-- http://url/to/file2.cfg content --
[buildout]
extends =
  http://url/to/sample1.cfg

-- http://url/to/file3.cfg content --
[buildout]
extends =
  sample1.cfg

In this case, http://url/to/sample1.cfg should be downloaded only once too.