Rietveld's "too large to download" patches.

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

Test bed: Rietveld issue 12980044
https://codereview.appspot.com/12980044/