Move worktree_branches call out of loop in ref update

[iczero]

worktree_branches opens every worktree as a Repository to resolve its HEAD. This is currently getting called in the loop over ref mappings, but I do not see why it needs to be. Moving it outside the loop results in a pretty significant performance increase (~10x in my case) for fetch when worktree(s) are present, dependent on how many refs exist in the repository and the value configured for object_store_slots. In my case, object_store_slots was set relatively high, resulting in slow drop for gix_odb::Store and hence gix::Repository.

[Byron]

Thanks so much, a great catch!

For a moment I was surprised that this worktree mapping is mutable, but it seems that it only sorts the worktree directories which indeed is idempotent in this context.