rfc support extern diff tool

Bug: 441120

I don't need this, but thought "why not try to fix" when it was working for years. I picked from the mentioned patch the "start extern tool" part. I have no idea if it is fine this way but it seems to work.

When there are no other objections beside the missing option, I will add that option in a way only be available when "Use seperate dialog.." on "Behavior" settings page is checked. Without this setting is this lambda code not called as it seems.