This mirrors the change in Omnibus:
https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/3559

In https://gitlab.com/gitlab-org/gitaly/merge_requests/1297, we
introduced a mechanism to prevent accidental deletion of objects from
object pools. The idea is that if an object becomes "dangling", meaning
there is no ref pointing to it, we just add a new ref to un-dangle
it. It turns out this can create too many refs.

If a pooled repository may contain many dangling references, a fetch
from the parent repository to one of its forks may take an
extraordinarily long time because git will attempt to run `git
for-each-ref` on every single ref in the pooled repository. This can
cause a simple fetch to take minutes.

To avoid this problem, we use a similar strategy that we employed in the
`git push` case
(https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/3364).  We
disable the fetching of refs from the alternates via the
`core.alternateRefsCommand` config option.