nix: Add backwardsCompatibleFetchgit

Needed so that we can use fetchgit with the branch nixos-16.03 as well as
current nixpkgs-unstable. It restores the behavior of nixos-16.03 which is
technically wrong since it removes all .git files.

Practically this issue has no big impact, so that we can keep it easily a few
months this way until we don't depend on building against nixos-16.03 anymore.

pkgs/python-packages-overrides.nix

@@ -14,6 +14,20 b' let'
       url =  http://www.repoze.org/LICENSE.txt;
     };
   };
+
+  # johbo: Interim bridge which allows us to build with the upcoming
+  # nixos.16.09 branch (unstable at the moment of writing this note) and the
+  # current stable nixos-16.03.
+  backwardsCompatibleFetchgit = { ... }@args:
+    let
+      origSources = pkgs.fetchgit args;
+    in
+    pkgs.lib.overrideDerivation origSources (oldAttrs: {
+      NIX_PREFETCH_GIT_CHECKOUT_HOOK = ''
+        find $out -name '.git*' -print0 | xargs -0 rm -rf
+      '';
+    });
+
 in
 
 self: super: {
@@ -96,7 +110,7 b' self: super: {'
   });
 
   py-gfm = super.py-gfm.override  {
-    src = pkgs.fetchgit {
+    src = backwardsCompatibleFetchgit {
       url = "https://code.rhodecode.com/upstream/py-gfm";
       rev = "0d66a19bc16e3d49de273c0f797d4e4781e8c0f2";
       sha256 = "0ryp74jyihd3ckszq31bml5jr3bciimhfp7va7kw6ld92930ksv3";
@@ -120,7 +134,7 b' self: super: {'
 
   Pylons = super.Pylons.override (attrs: {
     name = "Pylons-1.0.1-patch1";
-    src = pkgs.fetchgit {
+    src = backwardsCompatibleFetchgit {
       url = "https://code.rhodecode.com/upstream/pylons";
       rev = "707354ee4261b9c10450404fc9852ccea4fd667d";
       sha256 = "b2763274c2780523a335f83a1df65be22ebe4ff413a7bc9e9288d23c1f62032e";