Add a CreateFromTemplate() method to graph drivers, and use it instead
of a driver-oblivious diff/put method when we want to create a copy of
an image's top layer that has the same parent and which differs from the
original only in its ID maps.
This lets drivers that can quickly make an independent layer based on
another layer do something smarter than we were doing with the
driver-oblivious method. For some drivers, a native method is
dramatically faster.
Note that the driver needs to be able to do this while still exposing
just one notional layer (i.e., one link in the chain of layers for a
given container) to the higher levels of the APIs, so if the new layer
is actually a child of the template layer, that needs to remain a detail
that's private to the driver.
Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>
c073b43547