This fixes panics in the moby integration of buildkit when using cache.
Panics come from nil Differ in computeBlobChain which is called in GetRemote().
GetRemote() got moved from Worker to ImmutableRef during the lazy-pull refactor.
However, the ability to easily override GetRemote() got lost with that refactor.
This patch attempts to allow for the override while keeping changes minimal.
Signed-off-by: Tibor Vass <tibor@docker.com>
fad612e748