Introduce diff crate to unite BufferDiff and BufferChangeSet (#24392)

This is a refactoring PR that does three things:

- First, it introduces a new `diff` crate that holds the previous
contents of the `git::diff` module, plus the `BufferChangeSet` type
formerly of `project::buffer_store`. The new crate is necessary since
simply moving `BufferChangeSet` into `git::diff` results in a dependency
cycle due to the use of `language::Buffer` to represent the diff base in
`BufferChangeSet`.
- Second, it renames the two main types in the new diff crate:
`BufferDiff` becomes `BufferDiffSnapshot`, and `BufferChangeSet` becomes
`BufferDiff`. This reflects that the relationship between these two
types (immutable cheaply-cloneable "value" type + stateful "resource
type" with subscriptions) mirrors existing pairs like
`Buffer`/`BufferSnapshot`. References to "change sets" throughout the
codebase are updated to refer to "diffs" instead.
- Finally, it moves the base_text field of the new BufferDiff type to
BufferDiffSnapshot.

Release Notes:

- N/A

---------

Co-authored-by: maxbrunsfeld <max@zed.dev>

crates/worktree/src/worktree.rs

@@ -5386,7 +5386,7 @@ fn send_status_update_inner(
 
     let new_snapshot = state.snapshot.clone();
     let old_snapshot = mem::replace(&mut state.prev_snapshot, new_snapshot.snapshot.clone());
-    let changes = build_change_set(phase, &old_snapshot, &new_snapshot, &state.changed_paths);
+    let changes = build_diff(phase, &old_snapshot, &new_snapshot, &state.changed_paths);
     state.changed_paths.clear();
 
     status_updates_tx
@@ -5399,7 +5399,7 @@ fn send_status_update_inner(
         .is_ok()
 }
 
-fn build_change_set(
+fn build_diff(
     phase: BackgroundScannerPhase,
     old_snapshot: &Snapshot,
     new_snapshot: &Snapshot,