Revert "libgit2: 0.26.6 → 0.27.7"

This reverts commit cc50638176 because
it breaks cargo (again, see fca4fbeba9):

  $ cargo build
      Updating crates.io index
  Segmentation fault

pkgs/development/libraries/git2/0.27.nix

@@ -0,0 +1,36 @@
+{ stdenv, fetchFromGitHub, cmake, pkgconfig, python
+, zlib, libssh2, openssl, http-parser, curl
+, libiconv, Security
+}:
+
+stdenv.mkDerivation rec {
+  version = "0.27.7";
+  name = "libgit2-${version}";
+
+  src = fetchFromGitHub {
+    owner = "libgit2";
+    repo = "libgit2";
+    rev = "v${version}";
+    sha256 = "1q3mp7xjpbmdsnk4sdzf2askbb4pgbxcmr1h7y7zk2738dndwkha";
+  };
+
+  cmakeFlags = [ "-DTHREADSAFE=ON" ];
+
+  nativeBuildInputs = [ cmake python pkgconfig ];
+
+  buildInputs = [ zlib libssh2 openssl http-parser curl ]
+    ++ stdenv.lib.optional stdenv.isDarwin Security;
+
+  propagatedBuildInputs = stdenv.lib.optional (!stdenv.isLinux) libiconv;
+
+  enableParallelBuilding = true;
+
+  doCheck = false; # hangs. or very expensive?
+
+  meta = {
+    description = "The Git linkable library";
+    homepage = https://libgit2.github.com/;
+    license = stdenv.lib.licenses.gpl2;
+    platforms = with stdenv.lib.platforms; all;
+  };
+}