A way to configure a repository's selectivity after the fact

[glasserc]

I had some old data in my forge DB that was causing issues so I ended up deleting the Forge DB and re-adding the repository. When doing so, I chose "only individual topics", then changed my mind and wanted to fetch all topics. However, doing f n didn't seem to fetch all the topics. I looked at the code and it looks like it's due to a selective-p property on the repository, but I didn't see any user-facing way to toggle this property. Maybe this is because once a repository is not selective, it's not possible for it to become selective, because it already has a bunch of other data present? I ended up removing the repository using forge-remove-repository and then re-adding it, but I wonder if this could be more convenient.