R1:92abe3c8fb84

Extract scope line selection logic from the diff rendering engine so it can reasonably be iterated on

Summary:
Ref T13249. Ref T11738. See PHI985. Currently, we have a crude heuristic for guessing what line in a source file provides the best context.

We get it wrong in a lot of cases, sometimes selecting very silly lines like "{". Although we can't always pick the same line a human would pick, we //can// pile on heuristics until this is less frequently completely wrong and perhaps eventually get it to…