include hashsum for compressed patch file in .diff/Index

APT prefers to download files it has hashes for to check it actually got
the file it wanted instead of some man-in-the-middled file (by an attacker
or simply by a webportal returning a login mask for every request).
Giving it the hash of the compressed file saves APT from needing to
uncompress the patch before being able to verify its integrity.

Signed-off-by: David Kalnischkies <david@kalnischkies.de>
Signed-off-by: Joerg Jaspert <joerg@debian.org>