Git alias:

intercommit = !sh -c 'git show "$1" > .git/commit1 && git show "$2" > .git/commit2 && interdiff .git/commit[12] | less -FRS' -

Example:

git intercommit

If upstream applied a slightly modified patch, and we want to see the modifications, then we use the program interdiff of the patchutils package, and call "interdiff" between commits.