An operand under another was dropped as covered by it even when a
directory between them was named .zfs or was a symlink. The walk of
the outer operand enters neither, so the files under the inner operand
went unscanned and their records were deleted. walkReaches now reports
such an operand as not reached, so pruneRoots keeps it and walks it on
its own. README lists both cases with the others.
Judgement call: the new constant zfsDirName replaces the .zfs literal
in operandWarning and subdirJob too, because goconst rejects a third
copy.
Model: opus-5-5
sneak