When copying to a backend which has the PartialUploads feature flag
set and can Move files the file is copied into a temporary name first.
Once the copy is complete, the file is renamed to the real
destination.
This prevents other processes from seeing partially downloaded copies
of files being downloaded and prevents overwriting the old file until
the new one is complete.
This also adds --inplace flag that can be used to disable the partial
file copy/rename feature.
See #3770
Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>
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