open-in-gitweb: try to use the current branch's remote

We used to hardcode the remote called 'origin' which might not even
exist at all.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
open-in-gitweb: use GitHub whenever possible

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>