Changes is continuty's `fs.Change`-like method but leverages overlayfs's "upperdir" for computing the diff.

GetOverlayLayers returns all layer directories of an overlayfs mount.

GetUpperdir parses the passed mounts and identifies the directory that contains diff between upper and lower.

IsOverlayMountType returns true if the mount type is overlay-based.

WriteUpperdir writes a layer tar archive into the specified writer, based on the diff information stored in the upperdir.