git: Ingest ordered tags then ordered branches references

Closed Antoine R. Dumont requested to merge generated-differential-D6392-source into master

This should allow to gradually ingest the git history. If we don't deal with {master/main/HEAD} references first, it sounds highly plausible we won't have a mostly full connected graph at first (except for repositories with no tags). Started from tag v0.1.0, v0.2.0, etc...

If we couple this with the option to ingest repository with multiple packfiles, this should allow to ingest more incrementally large repositories (as it creates partial snapshots at each fetch/store loop).

Related to #3625 (closed) Depends on !136 (closed)

Migrated from D6392 (view on Phabricator)

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