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Closes #4461

Root cause: since #4407, fetchCache runs inside RunInLockedAccessMode, which creates the cache folder for its lock file. fetchCache then sees the folder, skips git clone --mirror, and runs git remote set-url in a non-repository.

Fix: if the folder exists but has no objects directory, initialise it with git init --bare and git remote add --mirror=fetch origin <url>. The existing remote update --prune then populates it. This follows the approach suggested in the issue. Existing caches are untouched.

Test Status

• dotnet tool restore and dotnet paket restore --group Main: succeeded
• dotnet build src/Paket.Core/Paket.Core.fsproj: succeeded, 0 errors
• Manually ran the init --bare / remote add --mirror=fetch / remote update --prune sequence in a folder containing paket.processlock. It worked, with a small difference: the sandbox's safe.bareRepository=explicit setting blocked the final remote update there.
• No automated test added: it needs a real git remote. Not run: unit and integration tests.

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