This script is supposed to make it easier to use git range-diff to compare how a branch has changed over time. When GitHub says "@mythmon force pushed from abcdef to 123456", you can run git-interdiff.sh abcdef 123456 to get a difference of each of those branches, based on where they departed from the main branch.
Note that if this script is named to git-interdiff and placed on your $PATH, you can run it as git interdiff.