Color git blame history by date in gray scale

[leana8959]

Hello,

I think it would be nice to color the Git blame window based on how recent the commit is, it would make finding out last change easier.

For example, we could make the most recent commit black and the older something gets the lighter the color gets.
Or maybe we could have something like "the last three changes to this file are red" or something like that?

Thank you for your time.

[tpope]

See #901 (comment).