Display (empty) when appropriate for the goto command

This is a fix for bug #11810. The original implementation of goto used
to display (empty patch) when a patch became empty during a push + merge
operation. This patch adds this feature again.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>
Acked-by: Karl Hasselström <kha@treskal.com>