Fetch missing Git bundle tracking refs - #71535

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Following up from #72759, which describes a different way to reach the same failing checkout: if a process is killed while a bundle version is being cloned, versions/<sha>/ is left with a .git that has no references and an empty working tree, and it is reused on every later initialization.

Your change looks like it recovers that state too, for a reason it was not written for:

• _has_version(repo, tracking_ref) cannot resolve tracking_ref in a repository with no refs, so it returns False and the fetch runs.
• git clone writes the remote before it transfers objects, so a leftover can still have remote.origin.url configured, pointing at the bundle's healthy local bare mirror. The fetch is local and repopulates refs/remotes/origin/*, after which the checkout resolves.

If you agree, would you like a regression test for that path added to this PR? It initializes a versioned bundle with a named branch such as main as tracking_ref, removes all loose and packed refs from versions/<sha>/.git, leaves HEAD pointing at a nonexistent branch, empties the working tree, verifies that origin remains configured, then initializes the same version again and asserts that both the expected version and the Dag files are restored. It fails on main and should pass with this change.

I have it written against the current TestGitDagBundle fixtures and can push it to your branch or post it here as a patch, whichever you prefer. Happy to leave it out if you would rather keep this PR scoped to #71388.

Thanks for the detailed follow-up — I agree the same recovery path covers the scenario in #72759, for the reasons you laid out (no resolvable refs -> fetch runs, and the leftover clone still has origin pointing at the local bare mirror).

A regression test for that path would be a valuable addition. Please post it here as a patch and I will apply it to this branch and update the PR description to reference #72759 alongside #71388.