fetchurl: add code to clean changing stuff from patches

Applied to github and cgit -looking URLs by default,
as the changes are likely to change the patches within a few months.
For example see #1983 and http://comments.gmane.org/gmane.linux.distributions.nixos/12815

Now I'm getting the same sha256 for github's *.diff and *.patch :-)

I don't think there's anything left now that would be changed by git/cgit updates. Comments welcome!