Visual selection in diff mode almost invisible

[rbong]

When diff is on, the selection is almost invisible in changed lines.

Example of block selection. You can still guess where it is.

And normal selection…

Not only the change of color of the text is almost non-existent, but also whitespaces are not changed at all. A solid background color for the complete selection would be nice.

🤖 This issue has been automatically copied from morhetz#237 🤖