Add a TurnDiffTracker to create a unified diff for an entire turn (#1770)

This lets us show an accumulating diff across all patches in a turn.
Refer to the docs for TurnDiffTracker for implementation details.

There are multiple ways this could have been done and this felt like the
right tradeoff between reliability and completeness:
*Pros*
* It will pick up all changes to files that the model touched including
if they prettier or another command that updates them.
* It will not pick up changes made by the user or other agents to files
it didn't modify.

*Cons*
* It will pick up changes that the user made to a file that the model
also touched
* It will not pick up changes to codegen or files that were not modified
with apply_patch

codex-rs/core/src/lib.rs

@@ -42,6 +42,7 @@ pub(crate) mod safety;
 pub mod seatbelt;
 pub mod shell;
 pub mod spawn;
+pub mod turn_diff_tracker;
 mod user_notification;
 pub mod util;