[META] This commit should be followed up by removing the comments and
assertions meant only to show that the change is correct.
`FindForkInGlobalIndex` only acts on BlockManager.
Note to reviewers: Since` FindForkInGlobalIndex` is always called with
`::ChainActive()` as its first parameter, it is possible to move
`FindForkInGlobalIndex` to `CChainState` and remove this `const CChain&`
parameter to instead use `m_chain`. However, it seems like the original
intention was for `FindForkInGlobalIndex` to work with _any_ chain, not
just the current active chain. Let me know if this should be changed.
This is a backport of core#20749 [5/17]
https://github.com/bitcoin/bitcoin/pull/20749/commits/b026e318c39f59a06e29f1b25c7f577e01b25ccb
Depends on D11173