If GitHub diff mode, load the 2 JS files needed
Ask message shown needed escaped slashes so they appear OK in dialog
No longer open a new file for older version, we display now in diff pane
Temp code added in comment block re getting content from GitHub for file
to show in diff pane. This isn't working at present and probably just
needs a tweak to make work
Slightly renamed vars for active lines to handle 2 panes