Git blame toggle button should indicate 3-states

Created by: Joelkw

The git blame extension button has the below on/off states, but it actually has three states: off, blame selected lines, blame all lines.

The button should tell me which option I have; right now, blame all lines and blame selected lines show the same "on" state.

Ideally, the button also implies that there are more than two states, so I don't just turn it on and assume that's all I can do. (For example, in UIs where I can see 3 states in a row and toggle between them. I think that's overkill here, but if there are ways to bring in that concept, that's a bonus.) That said, this is a secondary concern to "capture 3 states."