What's an interdiff in the first place?An interdiff a simple .txt file which contains all the different changes made between two patches(an old patch and a new patch).
Generating an interdiff is relatively easy. Following the steps below will make it easy for you to generate the interdiff.
sudo apt-get install patchutils;List of Issues:
There's a list of open issues here, whereby you can choose any of them and contribute to Drupal, by patching.
For the tutorial, we'll be using the issue with id 2639682. We'll generate the interdiff for 2639682-26.patch and 2639682-20.patch.
Before making the interdiff, you need to download both patches. An easy way to download the script is by using wget.
Now that you've download both patches, you can generate the interdiff with a single command.
interdiff 2639682-26.patch 2639682-20.patch > interdiff-26-20-27.txt