A -R source prune (--exclude/--max-size) must record the destination wire
path below the reconstructed prefix so --delete protects it; the parallel
root scan and the sequential skip path used the source path instead.
A general -R transfer places its files below the reconstructed prefix, so
marking the whole receive root as the delete scope deleted unrelated
sibling directories (data loss; rsync keeps them). Use the prefix itself
as the root marker when it is non-empty, in both the single-threaded and
multithreaded pipelines.
A trailing slash (or trailing '/.', or a bare '.') now lists the source's
immediate contents -- files transferred, subdirectories created empty --
without recursing, while a bare directory still sends only its own entry.
The -R prefix applies to the generated entries and to the root entry.
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