builtin/clone: allow remote helpers to detect repo

In 18c9cb75 (builtin/clone: create the refdb with the correct object format, 2023-12-12), we have changed git-clone(1) so that it delays creation of the refdb until after it has learned about the remote's object format. This change was required for the reftable backend, which encodes the object format into the tables. So if we pre-initialized the refdb with the default object format, but the remote uses a different object format than that, then the resulting tables would have encoded the wrong object format.

This change unfortunately breaks remote helpers which try to access the repository that is about to be created. Because the refdb has not yet been initialized at the point where we spawn the remote helper, we also don't yet have "HEAD" or "refs/". Consequently, any Git commands ran by the remote helper which try to access the repository would fail because it cannot be discovered.

This is essentially a chicken-and-egg problem: we cannot initialize the refdb because we don't know about the object format. But we cannot learn about the object format because the remote helper may be unable to access the partially-initialized repository.

Ideally, we would address this issue via capabilities. But the remote helper protocol is not structured in a way that guarantees that the capability announcement happens before the remote helper tries to access the repository.

Instead, fix this issue by partially initializing the refdb up to the point where it becomes discoverable by Git commands.

Closes Remote helpers hit undiscoverable repo during c... (#274 - closed).