Speed up the application of robust loss functions.

Since we added special handling for the case for rho[2] < 0,
the bulk of CorrectJacobian is pointless in the common case.
So add a simple one dimensional loop which rescales the Jacobian.
This speeds up this method immensely.

The robustification of a Jacobian gets speeded up by > 50%.

Change-Id: I97c4e897ccbb5521c053e1fb931c5d0d32f542c7
diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index c78364d..55caa05 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -106,8 +106,8 @@
 //   Jet<double, 2> y(1);  // Pick the 1st dual number for y.
 //   Jet<double, 2> z = f(x, y);
 //
-//   LG << "df/dx = " << z.a[0]
-//      << "df/dy = " << z.a[1];
+//   LOG(INFO) << "df/dx = " << z.a[0]
+//             << "df/dy = " << z.a[1];
 //
 // Most users should not use Jet objects directly; a wrapper around Jet objects,
 // which makes computing the derivative, gradient, or jacobian of templated