Clear pending staged/unstaged diff hunks hunks when writing to the git index fails (#26173)

Release Notes:

- Git Beta: Fixed a bug where discarding a hunk in the project diff view
performed two concurrent saves of the buffer.
- Git Beta: Fixed an issue where diff hunks appeared in the wrong state
after failing to write to the git index.

crates/sum_tree/src/tree_map.rs

@@ -70,6 +70,10 @@ impl<K: Clone + Ord, V: Clone> TreeMap<K, V> {
         self.0.insert_or_replace(MapEntry { key, value }, &());
     }
 
+    pub fn clear(&mut self) {
+        self.0 = SumTree::default();
+    }
+
     pub fn remove(&mut self, key: &K) -> Option<V> {
         let mut removed = None;
         let mut cursor = self.0.cursor::<MapKeyRef<'_, K>>(&());
@@ -157,6 +161,14 @@ impl<K: Clone + Ord, V: Clone> TreeMap<K, V> {
         self.0.iter().map(|entry| &entry.value)
     }
 
+    pub fn first(&self) -> Option<(&K, &V)> {
+        self.0.first().map(|entry| (&entry.key, &entry.value))
+    }
+
+    pub fn last(&self) -> Option<(&K, &V)> {
+        self.0.last().map(|entry| (&entry.key, &entry.value))
+    }
+
     pub fn insert_tree(&mut self, other: TreeMap<K, V>) {
         let edits = other
             .iter()