Handle patches applied as commits

Motivation for Request

Tests need to be able to extract diffs from Evergreen patches applied as commits.

Context

Evergreen would like to apply most regular patches as commits, breaking tests which rely on the previous behavior where Evergreen applied changes to the index without committing.

Description

We should look for a command that will get the diff no matter whether the changes have been committed, applied to the index, or exist only in the working tree.