convert: add config option for disabling ancestor parent checks When converting merge commits, convert checks if any of the parents are ancestors of any of the other parents. To do this, it builds an ancestor list for every commit in the repository. On large repos this can take a long time (30min+). Let's add an option for disabling this check to preserve performance. The downside of this is that it may create unnecessary parent connections when enabled (which is unfortunate, but not incorrect). To verify, I ran the convert tests with the flag enabled, and verified the graph changes were all just to add new parents that were ancestors of existing parents.