git loaders are getting oom-killed repeatedly in prod

Even though we try to limit the size of packfiles we're allowing to archive (to 4GiB), the git loaders are being repeatedly OOM-killed on the production VMs (that all have 12GiB of RAM and some swap space).

This seems to be blocking almost all archival of git repositories.

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Migrated from T3025 (view on Phabricator)