response to comment #2 : I personally go in the package source do "dch -i" , "dpkg-buildpackage -us -uc -rfakeroot -S" then "cd .." and finally "debdiff old.dsc new.dsc > your_patch.patch" .
response to comment #2 : I personally go in the package source do "dch -i" , "dpkg-buildpackage -us -uc -rfakeroot -S" then "cd .." and finally "debdiff old.dsc new.dsc > your_patch.patch" .