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/git_ui/src/project_diff.rs

@@ -2,6 +2,7 @@ use std::any::{Any, TypeId};
 
 use anyhow::Result;
 use collections::HashSet;
+use diff::BufferDiff;
 use editor::{scroll::Autoscroll, Editor, EditorEvent};
 use feature_flags::FeatureFlagViewExt;
 use futures::StreamExt;
@@ -11,7 +12,7 @@ use gpui::{
 };
 use language::{Anchor, Buffer, Capability, OffsetRangeExt, Point};
 use multi_buffer::{MultiBuffer, PathKey};
-use project::{buffer_store::BufferChangeSet, git::GitState, Project, ProjectPath};
+use project::{git::GitState, Project, ProjectPath};
 use theme::ActiveTheme;
 use ui::prelude::*;
 use util::ResultExt as _;
@@ -43,7 +44,7 @@ pub(crate) struct ProjectDiff {
 struct DiffBuffer {
     path_key: PathKey,
     buffer: Entity<Buffer>,
-    change_set: Entity<BufferChangeSet>,
+    diff: Entity<BufferDiff>,
 }
 
 const CONFLICT_NAMESPACE: &'static str = "0";
@@ -285,13 +286,13 @@ impl ProjectDiff {
                     let buffer = load_buffer.await?;
                     let changes = project
                         .update(&mut cx, |project, cx| {
-                            project.open_uncommitted_changes(buffer.clone(), cx)
+                            project.open_uncommitted_diff(buffer.clone(), cx)
                         })?
                         .await?;
                     Ok(DiffBuffer {
                         path_key,
                         buffer,
-                        change_set: changes,
+                        diff: changes,
                     })
                 }));
             }
@@ -312,15 +313,14 @@ impl ProjectDiff {
     ) {
         let path_key = diff_buffer.path_key;
         let buffer = diff_buffer.buffer;
-        let change_set = diff_buffer.change_set;
+        let diff = diff_buffer.diff;
 
         let snapshot = buffer.read(cx).snapshot();
-        let change_set = change_set.read(cx);
-        let diff_hunk_ranges = if change_set.base_text.is_none() {
+        let diff = diff.read(cx);
+        let diff_hunk_ranges = if diff.snapshot.base_text.is_none() {
             vec![Point::zero()..snapshot.max_point()]
         } else {
-            change_set
-                .diff_hunks_intersecting_range(Anchor::MIN..Anchor::MAX, &snapshot)
+            diff.diff_hunks_intersecting_range(Anchor::MIN..Anchor::MAX, &snapshot)
                 .map(|diff_hunk| diff_hunk.buffer_range.to_point(&snapshot))
                 .collect::<Vec<_>>()
         };