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/editor/src/scroll.rs

@@ -137,6 +137,7 @@ pub struct ScrollManager {
     hide_scrollbar_task: Option<Task<()>>,
     dragging_scrollbar: bool,
     visible_line_count: Option<f32>,
+    forbid_vertical_scroll: bool,
 }
 
 impl ScrollManager {
@@ -151,6 +152,7 @@ impl ScrollManager {
             dragging_scrollbar: false,
             last_autoscroll: None,
             visible_line_count: None,
+            forbid_vertical_scroll: false,
         }
     }
 
@@ -185,6 +187,9 @@ impl ScrollManager {
         workspace_id: Option<WorkspaceId>,
         cx: &mut ViewContext<Editor>,
     ) {
+        if self.forbid_vertical_scroll {
+            return;
+        }
         let (new_anchor, top_row) = if scroll_position.y <= 0. {
             (
                 ScrollAnchor {
@@ -224,6 +229,9 @@ impl ScrollManager {
         workspace_id: Option<WorkspaceId>,
         cx: &mut ViewContext<Editor>,
     ) {
+        if self.forbid_vertical_scroll {
+            return;
+        }
         self.anchor = anchor;
         cx.emit(EditorEvent::ScrollPositionChanged { local, autoscroll });
         self.show_scrollbar(cx);
@@ -298,6 +306,14 @@ impl ScrollManager {
             false
         }
     }
+
+    pub fn set_forbid_vertical_scroll(&mut self, forbid: bool) {
+        self.forbid_vertical_scroll = forbid;
+    }
+
+    pub fn forbid_vertical_scroll(&self) -> bool {
+        self.forbid_vertical_scroll
+    }
 }
 
 impl Editor {
@@ -334,6 +350,9 @@ impl Editor {
         scroll_delta: gpui::Point<f32>,
         cx: &mut ViewContext<Self>,
     ) {
+        if self.scroll_manager.forbid_vertical_scroll {
+            return;
+        }
         let display_map = self.display_map.update(cx, |map, cx| map.snapshot(cx));
         let position = self.scroll_manager.anchor.scroll_position(&display_map) + scroll_delta;
         self.set_scroll_position_taking_display_map(position, true, false, display_map, cx);
@@ -344,6 +363,9 @@ impl Editor {
         scroll_position: gpui::Point<f32>,
         cx: &mut ViewContext<Self>,
     ) {
+        if self.scroll_manager.forbid_vertical_scroll {
+            return;
+        }
         self.set_scroll_position_internal(scroll_position, true, false, cx);
     }