A snapshotDir tracks its current position as part of its state, so we need
to use it via pointer rather than value so that Readdirnames can actually
update that position, or else we'll just get stuck at position zero.
In practice this wasn't hurting anything because we only call Readdir once
on our snapshots, to read the whole directory at once. Still nice to fix
to avoid a gotcha for future maintenence, though.
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