R1:c5c53e277a28

Make line selection in source code views less fragile and more consistent

Summary:
Depends on D19347. Ref T13105. See PHI565. The "highlight lines" behavior is interacting poorly with the new blame element in Diffusion.

Make the behavior a little simpler and hopefully more robust.

Test Plan:
- Clicked commit/revision links in Diffusion, saw the links get followed instead of the lines highlighted.
- Highlighted lines in Diffusion, saw just the line/code highlight instead of the whole thing.