Explicitly disable auto gc on pool repositories

https://git-scm.com/docs/git-config#git-config-gcauto

Git will sometimes automatically run git gc. In GitLab we globally disable this with auto.gc=0 in Git config.

I think it's critical for object dedup that auto gc never runs in a pool repository because if the remotes are out of date that may lead to data loss. Considering that Gitaly does not own the global Git config, and because of the bad consequences if auto.gc gets set the wrong way, I think should consider explicitly setting gc.auto=0 in the local git config of pool repositories.