Add idempotency

[deitch]

When running through steps, first see if step previously took.

1. When shrinking filesystem, see if filesystem already is target size; if so, skip
2. When shrinking partition, see if partition already is target size; if so, skip
3. When creating new larger partitions, see if they match target ones; if so, skip
4. When copying contents to new partitions, first see if it already is copied; if so, skip