This moves the config loading closer to CLI args where --tool=<name> option
will be processed. The factory function are proxied through the command helper
so that the base_ignores can be attached there later.
I'm going to add a prefix resolution method to OpStore, but OpStore is
unrelated to the index. I think ObjectId, HexPrefix, and PrefixResolution can
be extracted to this module.
It seems better to have the caller pass the transaction description
when we finish the transaction than when we start it. That way we have
all the information we want to include more readily available.