This adds gc.root label to snapshots created with prepare and commit via
the CLI. WIthout this, created snapshots get immediately garbage
collected. There may be a better solution but this seems to be a solid
stop gap.
We may also need to add more functionality around snapshot labeling for
the CLI but current use cases are unclear.
Signed-off-by: Stephen J Day <stevvooe@gmail.com>
ed72059fac