Skip redundant commit pack download during clone when prefetch packs exist

[tyrielv]

Summary

When gvfs clone runs with a shared cache that already has prefetch packs (e.g., from a previous clone), defer the commit+trees pack download. This avoids redundantly re-downloading trees that are already available in the cache.

Changes

CloneVerb.cs — Clone optimization

• Before downloading the commit pack, check if the commit+root tree already exist locally (CommitAndRootTreeExists), or if usable prefetch packs are present in the shared cache (HasUsablePrefetchPacks).
• If prefetch packs exist but the commit is missing, defer the download. If CreateBranchWithUpstream or TryDownloadRootGitAttributes fails because the commit is needed, download on demand and retry.
• The existing checkout fallback ("unable to read tree" → force re-download) handles missing subtrees.

GitObjects.cs — Shared helpers

• IsUsablePrefetchPack(packPath) — checks a pack has a matching .idx and no .incomplete marker.
• HasUsablePrefetchPacks() — returns true if at least one usable prefetch pack exists.
• PrefetchStep.TryGetMaxGoodPrefetchTimestamp refactored to use IsUsablePrefetchPack instead of inline checks.

SharedCacheTests.cs — Functional tests

Two tests that manufacture specific shared-cache states via "cache surgery" (creating fake prefetch packs with git pack-objects):

1. SecondCloneSucceedsWithMissingTreesAndPrefetchPacks — commit+root tree present as loose objects, fake prefetch pack exists, but subtrees are missing. Verifies the checkout fallback re-downloads the commit pack.

2. SecondCloneWithPrefetchPacksButMissingCommit — target commit is completely absent but a fake prefetch pack exists. Exercises the deferred-download path (skippedCommitDownload) that falls back through CreateBranchWithUpstream/TryDownloadRootGitAttributes.

Scenarios

Cache state	Behavior
Commit+root tree in prefetch packs	CommitAndRootTreeExists → true → skip download
Prefetch packs exist, commit absent	Defer download → fallback on branch creation or gitattributes
Prefetch packs exist, commit+root tree present, subtrees missing	Skip download → checkout fails → fallback re-download
No prefetch packs	Download as before (no behavior change)