Sometimes you want to track a specific branch of nixpkgs (like
nixpkgs-unstable or some release branch) but you would like to apply some
unmerged pull requests or some other patches to that revision.

Note that overlays are not always sufficient in this case because they don't
work for NixOS modules and you're forced to duplicate code which already
exists in some commit.

In this case you could fork nixpkgs, create a branch based on your desired
upstream branch and cherry-pick the commits you need on top of that
branch. Then everytime you need to upgrade nixpkgs you rebase your branch on
the latest upstream branch that you're tracking. This process works but is a
bit involved and somewhat untransparent.

nixpkgs also supports a simpler and more transparent mechanism to apply some
patches to an existing revision. First you fetch your desired revision of
nixpkgs like for example:

  let
    nixpkgs =
      builtins.fetchTarball {
	url = "https://github.com/NixOS/nixpkgs/archive/86d58407a6bb8ef2e1a58ab50318b149ffd4feda.tar.gz";
	sha256 = "0a2lkvacn78nza9hlmdx9znp1my3c3jf3xyhk9ydw6lxa348b7sl";
      };

Then you import that revision and apply it to a list of patches, for example:

    patchedPkgs = import nixpkgs {
      patches = pkgs : [
	# NixOS#59950
	(pkgs.fetchpatch {
	  url = https://github.com/NixOS/nixpkgs/commit/1f770d20550a413e508e081ddc08464e9d08ba3d.patch;
	  sha256 = "1nlzx171y3r3jbk0qhvnl711kmdk57jlq4na8f8bs8wz2pbffymr";
	})
      ];
    };
  in patchedPkgs...

Note that `patches` should be a function from a nixpkgs set `pkgs` to a list
of patch files that can be applied to a nixpkgs directory tree. The `pkgs` set
should mainly be used to get the `fetchpatch` function from. Note that `pkgs`
is a nixpkgs set for the specified `localSystem` but without `overlays` and
`crossOverlays` defined.

Overlays are applied after patching which means you can have overlays which
can depend on the patched content.