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a1c540085f
Adds a new diff plugin that can import image layers in the block CIM format using the new block CIM layer writer added in hcsshim repo. This commit also makes another important change in the way a diff is applied when using CimFS based layer writers. Currently, the diff plugins call archive.Apply to apply a diff and pass a function (that can actually apply the diff) as an argument (via archive.ApplyOptions). This allows the callers to call archive.Apply with either a custom applier function or if the caller doesn't pass such a function archive.Apply uses the default naive diff applier. However, there is drawback to this approach. The applier function passed to the `archive.Apply` call needs to follow a specific signature. This signature expects it that all parent layers are represented as an array of strings. In cases like CimFS, we can't easily represent a set of layers as strings (unless we encode extra data in those strings in a hacky way). To get around this problem, the diff plugins for CimFS based layers, skip the archive.Apply call and directly call the layer writer instead. Signed-off-by: Amit Barve <ambarve@microsoft.com>