Allow to toggle git hunk diffs (#11080)

Part of https://github.com/zed-industries/zed/issues/4523

Added two new actions with the default keybindings

```
"cmd-'": "editor::ToggleHunkDiff",
"cmd-\"": "editor::ExpandAllHunkDiffs",
```

that allow to browse git hunk diffs in Zed:


https://github.com/zed-industries/zed/assets/2690773/9a8a7d10-ed06-4960-b4ee-fe28fc5c4768


The hunks are dynamic and alter on user folds and modifications, or
toggle hidden, if the modifications were not adjacent to the expanded
hunk.


Release Notes:

- Added `editor::ToggleHunkDiff` (`cmd-'`) and
`editor::ExpandAllHunkDiffs` (`cmd-"`) actions to browse git hunk diffs
in Zed

crates/language/src/buffer.rs

@@ -109,6 +109,7 @@ pub struct Buffer {
     deferred_ops: OperationQueue<Operation>,
     capability: Capability,
     has_conflict: bool,
+    diff_base_version: usize,
 }
 
 /// An immutable, cheaply cloneable representation of a fixed
@@ -304,6 +305,8 @@ pub enum Event {
     Reloaded,
     /// The buffer's diff_base changed.
     DiffBaseChanged,
+    /// Buffer's excerpts for a certain diff base were recalculated.
+    DiffUpdated,
     /// The buffer's language was changed.
     LanguageChanged,
     /// The buffer's syntax trees were updated.
@@ -643,6 +646,7 @@ impl Buffer {
             was_dirty_before_starting_transaction: None,
             text: buffer,
             diff_base,
+            diff_base_version: 0,
             git_diff: git::diff::BufferDiff::new(),
             file,
             capability,
@@ -872,6 +876,7 @@ impl Buffer {
     /// against the buffer text.
     pub fn set_diff_base(&mut self, diff_base: Option<String>, cx: &mut ModelContext<Self>) {
         self.diff_base = diff_base;
+        self.diff_base_version += 1;
         if let Some(recalc_task) = self.git_diff_recalc(cx) {
             cx.spawn(|buffer, mut cx| async move {
                 recalc_task.await;
@@ -885,6 +890,11 @@ impl Buffer {
         }
     }
 
+    /// Returns a number, unique per diff base set to the buffer.
+    pub fn diff_base_version(&self) -> usize {
+        self.diff_base_version
+    }
+
     /// Recomputes the Git diff status.
     pub fn git_diff_recalc(&mut self, cx: &mut ModelContext<Self>) -> Option<Task<()>> {
         let diff_base = self.diff_base.clone()?; // TODO: Make this an Arc
@@ -898,9 +908,10 @@ impl Buffer {
 
         Some(cx.spawn(|this, mut cx| async move {
             let buffer_diff = diff.await;
-            this.update(&mut cx, |this, _| {
+            this.update(&mut cx, |this, cx| {
                 this.git_diff = buffer_diff;
                 this.git_diff_update_count += 1;
+                cx.emit(Event::DiffUpdated);
             })
             .ok();
         }))