gitaly: Reintroduce migration to `[[git.config]]` stanzas

What does this MR do?

With commit 2727686d (gitaly: Migrate to inject Git configuration via config.toml, 2022-06-20), we have migrated the configuration for Git commands spawned by Gitaly from the system-level gitconfig file into Gitaly's config.toml via [[git.config]] stanzas. This change caused an incident because it change inadvertently fixed a configuration error: because we had globally been setting core.alternateRefsCommand=exit 0# in the gitconfig, Gitaly mistakenly ignored all alternate references in object pools. This is wrong though when calculating the repository size, where we need to consider alternate references in order to correctly calculate the size of objects that are only contained in the repository itself.

But while the migration to [[git.config]] stanzas fixed this bug, it surfaced that the current implementation of how this is done in Gitaly is extremely inperformant in large repositories with many references. And this inefficiency ultimately end to starving the node of resources, which led to the revert of this change via a42c5611 (Merge branch 'revert-fe00a13e' into 'master', 2022-06-20).

Gitaly has since fixed this performance edge case so that we shouldn't hit the same bug this time. So let's reintroduce the migration to the new way of configuring Git so that we can also fix the bug about repo size calculations.

Note that this commit is a simple revert. This time around we'll also introduce a new flag to opt-in to this migration so that we can first excessively test it in staging without also impacting production. To keep this as a clean revert though this change will be done in the subsequent commit.

Related issues

• Omnibus issue to migrate to [[git.config]]: #6800 (closed)
• Incident: gitlab-com/gl-infra/production#7284 (closed)
• FCL epic: &8287 (closed)
• Performance regression fix: gitaly#4313 (closed)

Checklist

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