Add diff support for new docker save format

[sharifelgamal]

The standard docker image format looks like something like

80484ae1c6bf4188800d727446232d1f0f03ae27104878c83d21794291283bf0/
80484ae1c6bf4188800d727446232d1f0f03ae27104878c83d21794291283bf0/VERSION
80484ae1c6bf4188800d727446232d1f0f03ae27104878c83d21794291283bf0/json
80484ae1c6bf4188800d727446232d1f0f03ae27104878c83d21794291283bf0/layer.tar
8176e035de519557279f4a0a46d8b72bbab71565867311af57ae00381ca18314.json
b02b500eecbef83c9c566b28ae72270d5cf4d38fb37692c436d603a17f41dd0e/
b02b500eecbef83c9c566b28ae72270d5cf4d38fb37692c436d603a17f41dd0e/VERSION
b02b500eecbef83c9c566b28ae72270d5cf4d38fb37692c436d603a17f41dd0e/json
b02b500eecbef83c9c566b28ae72270d5cf4d38fb37692c436d603a17f41dd0e/layer.tar
manifest.json
repositories

But there's an alternate format, where the layer tars are gzipped and renamed to the sha of its contents. It looks something like

manifest.json
sha256:1ec95b5530f7966ad7d3dde70799911c7f8c2e4fb1aebdd430a4895b7c661d7e
sha256:b51579ef26c457b63f2b70dcbc80518c28c5b9a0b075733669ff534ae8ac9e11
sha256:fe6ba4acf23d5ef92b0eafa2a92f289440dd8c80f35d47db4f9073fd7c681e7b

This new format currently breaks container-diff diff. Two otherwise identical images, saved in each format will show up as having apt differences.