Allow to download "too large to download" patchsets.

Avoid the 404 error on patchsets that are "too large".
Since patch download links are available for individual
files, we donwload them and concatenate them to obtain
the complete patch. curl allows us to do this in one go.

Workaround for Rietveld issue 196:
http://code.google.com/p/rietveld/issues/detail?id=196