I generated the source package with this command
backportpackage -s utopic -d trusty -w . tango
and then the debdiff with
debdiff tango_8.1.2c+dfsg-5.dsc tango_8.1.2c+dfsg-5~ubuntu14.04.1.dsc > tango_8.1.2c+dfsg-5~ubuntu14.04.1.debdiff
hope it is ok
I generated the source package with this command
backportpackage -s utopic -d trusty -w . tango
and then the debdiff with
debdiff tango_8. 1.2c+dfsg- 5.dsc tango_8. 1.2c+dfsg- 5~ubuntu14. 04.1.dsc > tango_8. 1.2c+dfsg- 5~ubuntu14. 04.1.debdiff
hope it is ok