When shrinking a volume by a certain size, instead of typing
vol-resize volume 1G --delta --shrink
we allow the convience of specifying a negative value:
vol-resize volume -1G --delta --shrink
getting the same results with one more character.
A negative value only makes sense as a delta. Imply the
--delta parameter if the value is negative.
Still require --shrink, because the operation is potentially
destructive.
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