operations: stop --copy-dest replacing files with --immutable

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When the source matched a file in --copy-dest, copyDest server-side
copied it over an existing destination which differed, so sync, copy
and copyto with --copy-dest could modify a file --immutable should have
protected.

Leave such a destination for the caller, which rejects it with
ErrorImmutableModified as for any other modified file.

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