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/git/src/diff.rs

@@ -5,7 +5,7 @@ use text::{Anchor, BufferId, BufferSnapshot, OffsetRangeExt, Point};
 pub use git2 as libgit;
 use libgit::{DiffLineType as GitDiffLineType, DiffOptions as GitOptions, Patch as GitPatch};
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum DiffHunkStatus {
     Added,
     Modified,
@@ -173,7 +173,8 @@ impl BufferDiff {
         })
     }
 
-    pub fn clear(&mut self, buffer: &text::BufferSnapshot) {
+    #[cfg(test)]
+    fn clear(&mut self, buffer: &text::BufferSnapshot) {
         self.last_buffer_version = Some(buffer.version().clone());
         self.tree = SumTree::new();
     }