context: introduce an `isintroducedafter` method and use it in copies Right now, copy tracing make effort to not traverse the graph too much to save performance. It uses a "limit" acting as a floor revision past which data are no longer relevant to the current copy tracing. However, to enforce this limit, it does a call to `filectx.rev()` and that call can trigger a graph traversal on its own. That extra graph traversal is unaware of the current limit and can become very expensive. That cost is increased by the nature of work done in adjust link rev, we are not only walking down the graph, we are also checking the affected file for each revision we walk through. Something significantly more expensive than the walk itself. To work around this we need to make the `filectx` operation aware of the current limit. The first step is to introduce a dedicated method: `isintroducedafter`. We'll then rework that method logic to stop traversal as soon as possible.