dolt_merge now impossible with conflicts

Due to other recent changes in transaction handling semantics, it's now impossible to resolve merge conflicts in Doltgres, because the `dolt_merge()` function returns an error in that case, which automatically rolls back any in-progress transactions before conflicts can be addressed.

To fix this, we need to change this behavior (dolt_merge() cannot return an error when conflicts are present) at least for Doltgres, and potentially for Dolt as well.

Start at the dolt_merge() entry point and trace how transaction handling treats conflicts in Doltgres. Determine how to preserve an in-progress transaction when conflicts are present, then verify whether the behavior also needs to change for Dolt; done means conflicts can be addressed without the merge automatically rolling back the transaction.

Written by the indexing model from the issue text.