Summary:
We need a way for mergedrivers to be able to read ancestor data (to properly detect a backwards merge, such as during `hg backout`).
Passing it directly is tricky since passing a new kwarg to functions without `**args` raises a `TypeError`. We'd have to update the hook framework to catch this `TypeError` and re-run the hook without the kwarg, which would be hard to do cleanly.
Rather, we can just add this as a transient property on `mergestate`, which seems like an appropriate place for it. Any callers trying to read this when it hasn't been set will get an exception. But there aren't any other users of this field since it hasn't existed before.
Reviewed By: quark-zju
Differential Revision: D12827614
fbshipit-source-id: 48dc84f1e60232625c8f2a73e81b5bc223c22bb3
