Merge #2630: add and use fetchpatch

fetchpatch is fetchurl that determinizes the patch.
Some parts of generated patches change from time to time, e.g. see #1983 and
http://comments.gmane.org/gmane.linux.distributions.nixos/12815
Using fetchpatch should prevent the hash from changing.

Conflicts (auto-solved):
	pkgs/development/libraries/haskell/gitit/default.nix