The delta file is a compressed tarball, containing the following files:

type
	Type of file this is a delta for ("tar", "gz", or "bz2").
version
	The meaning depends on the "type" value.
sha256sum
	hex-encoded sha256sum hash of the original tarball used to verify the
	integrity of existing tarballs. Optional; if not present, the stored
	tarball will be checked out, hashed, and that hash will be used
	instead.

It also contains files depending on the "type" value.


For tar files, it contains:

version
	Currently either "2" (or "2.0") (xdelta based) or "3" (xdelta3 based).
manifest
	List of all files in the tarball, as output by `tar t`.
	Used to order files correctly when rebuilding it.
delta
	xdelta between the generated tarball and the original tarball.
wrapper
	Encapsulated delta file for the .gz, .bz2 or .xz wrapper for the
	tarball. Optional, if not present a pristine .gz won't be generated.


For gz files, wrapper contains:

version
	Either "2.0" (no delta), "3.0" (xdelta based) or "4" (xdelta3 based).
params
	Parameters to pass to zgz. ("-n 9", "-M", "--rsyncable")
timestamp
	Timestamp of the original input file, in seconds from epoch.
filename
	Filename of the original input file.
delta
	xdelta between the generated gz file and the original gz file.
	(Optional; needs version >= "3.0".)


For bzip2 files the wrapper contains:

version
	Currently only "2.0".
params
	Typically, only the compression level is needed (4th byte of the
	compressed file), and its matching parameter stored: -N.
	In some cases a -bN parameter is detected and stored.
program
	Program used to compress. Almost everytime, it is bzip2 (or another
	implementation producing bit-identical results). pbzip2 might also be
	detected, but several parameters might interfere (-r, -pN).

	It may also be zgz (the params will include --old-bzip2 in this
	case).


For xz files, the wrapper contains:

version
	Currently only "2.0".
params
	Typically, only the compression level is needed.
program
	Program used to compress. Almost everytime, it is xz.
