feat: diff tree to index

This is the primitive needed to complete `gix-status`.

Author: Byron

gix-status/src/lib.rs

@@ -2,10 +2,14 @@
 //! of the repository state, like comparisons between…
 //!
 //! * index and working tree
-//! * index and tree
-//! * find untracked files
+//! * tree and index
 //!
-//! While also being able to check check if the working tree is dirty, quickly.
+//! While also being able to check if the working tree is dirty, quickly.
+//!
+//! ### Difference to `gix-diff`
+//!
+//! Technically, `status` is just another form of diff between different kind of sides, i.e. an index and a working tree.
+//! This is the difference to `gix-diff`, which compares only similar items.
 //!
 //! ### Feature Flags
 #![cfg_attr(