Don't autosave unmodified buffers (#32626)

Closes https://github.com/zed-industries/zed/issues/12091

Proper redo of https://github.com/zed-industries/zed/pull/32603

Release Notes:

- Fixed formatting effects not triggered when saving unmodified
singleton buffers

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Cole Miller <m@cole-miller.net>

crates/fs/src/fs.rs

@@ -897,6 +897,7 @@ struct FakeFsState {
     buffered_events: Vec<PathEvent>,
     metadata_call_count: usize,
     read_dir_call_count: usize,
+    path_write_counts: std::collections::HashMap<PathBuf, usize>,
     moves: std::collections::HashMap<u64, PathBuf>,
     home_dir: Option<PathBuf>,
 }
@@ -1083,6 +1084,7 @@ impl FakeFs {
                 events_paused: false,
                 read_dir_call_count: 0,
                 metadata_call_count: 0,
+                path_write_counts: Default::default(),
                 moves: Default::default(),
                 home_dir: None,
             })),
@@ -1173,6 +1175,8 @@ impl FakeFs {
         recreate_inode: bool,
     ) -> Result<()> {
         let mut state = self.state.lock();
+        let path_buf = path.as_ref().to_path_buf();
+        *state.path_write_counts.entry(path_buf).or_insert(0) += 1;
         let new_inode = state.get_and_increment_inode();
         let new_mtime = state.get_and_increment_mtime();
         let new_len = new_content.len() as u64;
@@ -1727,6 +1731,17 @@ impl FakeFs {
         self.state.lock().metadata_call_count
     }
 
+    /// How many write operations have been issued for a specific path.
+    pub fn write_count_for_path(&self, path: impl AsRef<Path>) -> usize {
+        let path = path.as_ref().to_path_buf();
+        self.state
+            .lock()
+            .path_write_counts
+            .get(&path)
+            .copied()
+            .unwrap_or(0)
+    }
+
     fn simulate_random_delay(&self) -> impl futures::Future<Output = ()> {
         self.executor.simulate_random_delay()
     }