From NovaOrdis Knowledge Base
Changes in the Working Directory that are Not Staged
Changes Staged for Next Commit
This shows the difference between the index and the last commit.
git diff --cached|--staged
Changes in Working Directory since Last Commit
git diff HEAD
Differences between Two Branches
git diff develop..task/my-branch path/to/folder
It seems the order in which the branches are specified does not matter.
For the entire repository:
git diff develop..task/my-branch .