libgit2: add 0.21.x version as it's incompatible with the new 0.23

pkgs/desktops/gnome-3/3.16/default.nix

···
 
   gitg = callPackage ./misc/gitg { 
     webkitgtk = webkitgtk24x;
+    libgit2 = pkgs.libgit2_0_21;
   };
 
   libgda = callPackage ./misc/libgda { };
 
   libgit2-glib = callPackage ./misc/libgit2-glib {
-    libgit2 = pkgs.libgit2.override { libssh2 = null; };
+    libgit2 = pkgs.libgit2_0_21.override { libssh2 = null; };
   };
 
   libmediaart = callPackage ./misc/libmediaart { };

pkgs/development/libraries/git2/0.21.nix

···
+{stdenv, fetchurl, cmake, zlib, python, libssh2, openssl, http-parser}:
+
+stdenv.mkDerivation rec {
+  version = "0.21.2";
+  name = "libgit2-${version}";
+
+  src = fetchurl {
+    name = "${name}.tar.gz";
+    url = "https://github.com/libgit2/libgit2/tarball/v${version}";
+    sha256 = "0icf119lhha96rk8m6s38sczjr0idr7yczw6knby61m81a25a96y";
+  };
+
+  cmakeFlags = "-DTHREADSAFE=ON";
+
+  nativeBuildInputs = [ cmake python ];
+  buildInputs = [ zlib libssh2 openssl http-parser ];
+
+  meta = {
+    description = "the Git linkable library";
+    homepage = http://libgit2.github.com/;
+    license = stdenv.lib.licenses.gpl2;
+    platforms = with stdenv.lib.platforms; all;
+  };
+}

pkgs/top-level/all-packages.nix

···
 
   libgit2 = callPackage ../development/libraries/git2 { };
 
+  libgit2_0_21 = callPackage ../development/libraries/git2/0.21.nix { };
+
   glew = callPackage ../development/libraries/glew { };
 
   glfw = glfw3;