Tooltips for Git and GitHub buttons in status bar

While conducting a user interview yesterday, @kuychaco pointed out that there are no tooltips for the Git and GitHub buttons in the status bar. The branch management button also does not have a hover tooltip (the menu is itself a tooltip, but ehh, that's more of an implementation detail.)

Tooltips that trigger on hover might help users understand what these buttons do before they click and or improve discoverability. Or they might just annoy our users and take up valuable screen real estate. Seems worth investigating.