Use try_lock in diff_handle for Diff gutter (#11092)

* Use `try_lock` in `diff_handle` for Diff gutter
- Add the method `try_load() -> Option<Diff>` to `DiffHandle`, using `try_lock` to avoid deadlocks.
- Use said method in `gutter.rs diff()`, which will use a blank `GutterFn` instead when met with a locked `Diff`.

* Revert changes

* Replace `Mutex` with `RwLock` in `Diff`

---------

Co-authored-by: Kaniel Kirby <pirate7007@runbox.com>

helix-vcs/src/diff/worker.rs

@@ -4,7 +4,7 @@ use std::sync::Arc;
 
 use helix_core::{Rope, RopeSlice};
 use imara_diff::intern::InternedInput;
-use parking_lot::Mutex;
+use parking_lot::RwLock;
 use tokio::sync::mpsc::UnboundedReceiver;
 use tokio::sync::Notify;
 use tokio::time::{timeout, timeout_at, Duration};
@@ -21,7 +21,7 @@ mod test;
 
 pub(super) struct DiffWorker {
     pub channel: UnboundedReceiver<Event>,
-    pub diff: Arc<Mutex<DiffInner>>,
+    pub diff: Arc<RwLock<DiffInner>>,
     pub new_hunks: Vec<Hunk>,
     pub diff_finished_notify: Arc<Notify>,
 }
@@ -73,7 +73,7 @@ impl DiffWorker {
     /// `self.new_hunks` is always empty after this function runs.
     /// To improve performance this function tries to reuse the allocation of the old diff previously stored in `self.line_diffs`
     fn apply_hunks(&mut self, diff_base: Rope, doc: Rope) {
-        let mut diff = self.diff.lock();
+        let mut diff = self.diff.write();
         diff.diff_base = diff_base;
         diff.doc = doc;
         swap(&mut diff.hunks, &mut self.new_hunks);