What it looks like in emacs -Q.
Before this patch, dark theme users cannot distinguish the line from the background even though hl-line-mode is enabled. Personally I can't.
Furthermore, users must set hl-line-sticky-flag to t to enable highlight in all windows or move the point to the source file buffer, otherwise only the arrow in fringe is helpful for which line program run to. It's too restricted in such a situation.
So I propose to add a new overlay of current line to make gud more friendly and deprecate the old one.
Here is my patch. :-)
