Fix a couple of diff hunk issues (#24913)

- Fix hunks losing their expandedness when staged or unstaged
- Disable collapsing hunks in the project diff editor

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/multi_buffer/src/multi_buffer.rs

@@ -281,13 +281,28 @@ enum DiffTransform {
     },
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Copy, Debug)]
 struct DiffTransformHunkInfo {
     excerpt_id: ExcerptId,
     hunk_start_anchor: text::Anchor,
     hunk_secondary_status: DiffHunkSecondaryStatus,
 }
 
+impl Eq for DiffTransformHunkInfo {}
+
+impl PartialEq for DiffTransformHunkInfo {
+    fn eq(&self, other: &DiffTransformHunkInfo) -> bool {
+        self.excerpt_id == other.excerpt_id && self.hunk_start_anchor == other.hunk_start_anchor
+    }
+}
+
+impl std::hash::Hash for DiffTransformHunkInfo {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.excerpt_id.hash(state);
+        self.hunk_start_anchor.hash(state);
+    }
+}
+
 #[derive(Clone)]
 pub struct ExcerptInfo {
     pub id: ExcerptId,
@@ -2361,7 +2376,7 @@ impl MultiBuffer {
         self.expand_or_collapse_diff_hunks(vec![Anchor::min()..Anchor::max()], true, cx);
     }
 
-    pub fn all_diff_hunks_expanded(&mut self) -> bool {
+    pub fn all_diff_hunks_expanded(&self) -> bool {
         self.all_diff_hunks_expanded
     }
 
@@ -2401,6 +2416,9 @@ impl MultiBuffer {
         expand: bool,
         cx: &mut Context<Self>,
     ) {
+        if self.all_diff_hunks_expanded && !expand {
+            return;
+        }
         self.sync(cx);
         let mut snapshot = self.snapshot.borrow_mut();
         let mut excerpt_edits = Vec::new();
@@ -2986,7 +3004,7 @@ impl MultiBuffer {
                     let was_previously_expanded = old_expanded_hunks.contains(&hunk_info);
                     let should_expand_hunk = match &change_kind {
                         DiffChangeKind::DiffUpdated { base_changed: true } => {
-                            self.all_diff_hunks_expanded
+                            self.all_diff_hunks_expanded || was_previously_expanded
                         }
                         DiffChangeKind::ExpandOrCollapseHunks { expand } => {
                             let intersects = hunk_buffer_range.is_empty()