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

pkgs/build-support/fetchpatch/default.nix

@@ -0,0 +1,22 @@
+# This function downloads and normalizes a patch/diff file.
+# This is primarily useful for dynamically generated patches,
+# such as GitHub's or cgit's, where the non-significant content parts
+# often change with updating of git or cgit.
+# stripLen acts as the -p parameter when applying a patch.
+
+{ fetchurl, patchutils }:
+{ stripLen ? 0, ... }@args:
+
+fetchurl ({
+  postFetch = ''
+    tmpfile="$TMPDIR/${args.sha256}"
+    "${patchutils}/bin/lsdiff" "$out" \
+      | sort -u | sed -e 's/[*?]/\\&/g' \
+      | xargs -I{} \
+        "${patchutils}/bin/filterdiff" \
+        --include={} \
+        --strip=${toString stripLen} \
+        --clean "$out" > "$tmpfile"
+    mv "$tmpfile" "$out"
+  '';
+} // args)