This moves Driver.Size() to Differ.DiffSize(), removing the empty
implementations in devmapper and dummy, and renaming the one in aufs.
Then we fall back to a container.Changes() implementation in the non-aufs
case.
This replaces the current version with the latest version from the
dm branch. Advantages in this version:
We only scan each directory tree once, which means less i/o and less chance
of container changes inbetween the two scans causing inconsistencies.
We avoid comparing some fields for change detection:
* Inode
* size-in-blocks
These can change during a copy operation (e.g. in the dummy backend)
without needing to actually reflect a change in content or metadata.
* Ctime
Any copy operation will create a new Ctime value, and there is no
API to change it to the "source" value.
* size for directories
The size of a directory doesn't have to be the same just because you
recreated the same content as another director. Internal details in
the filesystem may make these different with no "real" change.
Andrews Medina