When the source of the copy operation is the root directory, we should
neither append it to the destination path on FTS_D nor trim it back off
on FTS_DP.
PR: 291132
MFC after: 3 days
Fixes: 82fc0d09e8 ("cp: Partly restore symlink folllowing.")
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53863
Dag-Erling Smørgrav
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