Load Git trees lazily on checkout

[zanieb]

Exploring #3950

[zanieb]

Turns out this is still really hard. Some weird behavior due to our local clone, I think?

[zanieb]

I suspect that we are doing a dynamic fetch and the promisor is the sparse local tree?

Since almost all Git code currently expects any referenced object to be present locally and because we do not want to force every command to do a dry-run first, a fallback mechanism is added to allow Git to attempt to dynamically fetch missing objects from promisor remotes.

When the normal object lookup fails to find an object, Git invokes promisor_remote_get_direct() to try to get the object from a promisor remote and then retry the object lookup. This allows objects to be "faulted in" without complicated prediction algorithms.

Dynamic object fetching will only ask promisor remotes for missing objects. We assume that promisor remotes have a complete view of the repository and can satisfy all such requests.

Which fails since git/git@301f1e3