Chomp ../ from beginging of filenames.

In order to have as canonical as possible paths, chomp ../ from filenames and
recalculate base.

src/zc/buildout/buildout.py

@@ -1385,6 +1385,11 @@ def _open(base, filename, seen, dl_options, override, downloaded):
 
     Recursively open other files based on buildout options found.
     """
+    counter = 0
+    while filename.startswith('../'):
+      filename = filename.replace('../', '', 1)
+      counter += 1
+    base = base.rsplit('/', counter)[0]
     _update_section(dl_options, override)
     _dl_options = _unannotate_section(dl_options.copy())
     newest = _convert_bool('newest', _dl_options.get('newest', 'false'))