Defaults to the value of Acquire::PDiffs which is true by default.
Can be overridden per-source by the sources.list option of the same
name. See the documentation for both of these for details.
+* By-Hash: controls if apt will try to use an URI constructed from
+ a hashsum of the file to download. See the documentation for config
+ option Acquire::By-Hash and sources.list option By-Hash for details.
* CompressionTypes: The default value is a space separated list of
compression types supported by apt (see Acquire::CompressionTypes).
You can set this option to prevent apt from downloading a compression