https://go-review.googlesource.com/c/39932/ handles relative symlinks.
But that change is incomplete.
We also have to handle relative symlinks starting with slash too.

Fixes #19937

Change-Id: I50dbccbaf270cb48a08fa57e5f450e5da18a7701
Reviewed-on: https://go-review.googlesource.com/40410Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>