Disable hooks when fetching

This MR fixes performance issues we have started to hit because we now execute reference-transaction hooks for each updated reference. As git fetch will create a transaction per updated reference while there is no support for the equivalent of git fetch --atomic, this doesn't scale well with our usecases of FetchIntoObjectPool and FetchInternalRemote, which both may update hundreds of thousands of references.

As a stop-gap measure, this MR thus disables hooks in this context.