PSA: cp -r behaves differently on linux and macOS

Huh, maybe that was the original inspiration for the equivalent behavior in rsync.

> A trailing slash on the source changes this behavior to avoid creating an additional directory level at the destination. You can think of a trailing / on a source as meaning "copy the contents of this directory" as opposed to "copy the directory by name", but in both cases the attributes of the containing directory are transferred to the containing directory on the destination.