In brief
git diff-tree compares contents and modes in Git trees. With one commit, its usual form compares that commit with its parents.
Useful to know
git diff-tree compares contents and modes in Git trees. With one commit, its usual form compares that commit with its parents. [1]
It helps tooling list historical changes without relying on current working files. The chosen view matters for a merge commit. Establish whether the comparison should use a particular parent or a combined result.
Sources
For further detail and to check the information: