Whenever you need to work on an arbitrary Debian™ package, you can check it right into Git with one command:

  gbp import-dsc apt:package
  cd package
  git branch debian

This uses apt-get to download the source package, puts the orig tarball on the upstream-branch and the Debian™ changes on the debian-branch (by default master). The second command creates a branch called debian. Now you can easily modify the package, revert changes you made, create other branches for testing, see what changes you made, etc. When finished, just do

  git commit -a
  git diff debian --

to get a nice patch that can be submitted to the Debian™ BTS. You can also fetch the source package from a URL:

  gbp import-dsc http://mentors.debian.net/debian/pool/main/i/ipsec-tools/ipsec-tools_0.7.3-9.dsc

The import works incrementally; you can import new versions on top of already imported ones for e.g. easy review of changes.