add back blame-on-current-line behavior to git-extras

Created by: sqs

Sourcegraph used to show blame info for the selected line. Now it only lets you toggle blame on/off for the entire file, and it shows the blame info for the first line of each hunk. Many people liked the previous behavior, and we want to bring it back.

Blocked on #2026 (closed) to add the extension APIs necessary for this.