fetchpatch fails to patch

[danbst]

Issue description

I'm trying to apply patch https://github.com/NixOS/nixpkgs/pull/18419.patch to my local nixpkgs with fetchpatch.

It fails, because fetchpatch reorganises diffs in a wrong way. I have 1 commit that creates a default.nix file, and 1 that changes is. Patching using https://github.com/NixOS/nixpkgs/pull/18419.patch works because changes have same order. Patching using fetchpatch fails, because first diff is changing default.nix, and next is creating default.nix.

Not sure whether this is a bug or a normal condition, because comment explicitly says that

This is primarily useful for dynamically generated patches,
such as GitHub's

ping @vcunat

Steps to reproduce

let
  pkgsWithPatches = tarballURL: patches: parentPkgs: parentPkgs.stdenv.mkDerivation {
    name = "nixpkgs-patched";
    src = fetchTarball tarballURL;
    phases = [ "unpackPhase" "patchPhase" ];
    patches = map parentPkgs.fetchpatch patches;
    postPatch = ''
      mkdir $out && cp -r * $out/
      echo "16.09" > $out/.version
      echo "16.09pre.custom" > $out/.version-suffix
    '';
  };
  URL = "https://github.com/NixOS/nixpkgs-channels/archive/562027e6c211ab9287850aa9efeb8da8c894de8c.tar.gz"; # 28.08.2016
  patches = [
      { meta.description = "update tomcat";
        url = "https://github.com/NixOS/nixpkgs/pull/18419.patch";
        sha256 = "0mxf8csah6yx66q37va668wikibcnzclppf73aww7h9i3n7lx2zz";
      }
  ];
  repo = pkgsWithPatches URL patches (import (fetchTarball URL) {});
in repo

[danbst]

Crap, just after I've filled an issue, I found out that .diff works for me (instead of .patch).