Diff lines should preserve foreground colors

[jerryyin]

Description

Right now diff lines:

• Only have solid background color
• Does not have foreground color so syntax highlighting is off

This is so that lines retain their syntax highlighting in the diff mode, while the backgrounds are highlighted.

Example

To illustrate the point, solarized color scheme allow diff mode preserve the foreground color while having non-eye-popping color from background, gruvbox should be able to do it too.

For solarized implementation refer to this commit.