Log filesystem watcher lost sync only once per event (#57675)

On Linux, we create a filesystem watcher recursively for every subdir.
When we get "fs watcher lost sync" events, we used to log it for every
child dir, which could results in thousands messages. This becomes
problematic when we get into a state where we get those events
repeatedly (this larger issue is to be addressed separately).

Now we log one message per parent.

Partially addresses #57422, #57042, FR-18

Release Notes:

- N/A

crates/fs/src/fs_watcher.rs

@@ -423,7 +423,6 @@ fn push_notify_event(
         .collect::<Vec<_>>();
 
     if event.need_rescan() {
-        log::warn!("filesystem watcher lost sync for {watched_root:?}; scheduling rescan");
         path_events.retain(|path_event| path_event.path != watched_root);
         path_events.push(PathEvent {
             path: watched_root.to_path_buf(),
@@ -750,6 +749,12 @@ fn handle_event(mode: WatcherMode, event: Result<notify::Event, notify::Error>)
 
     match event {
         Ok(event) => {
+            if event.need_rescan() {
+                log::warn!(
+                    "filesystem watcher lost sync for {mode:?}; scheduling rescans for {} registrations",
+                    callbacks.len()
+                );
+            }
             for callback in callbacks {
                 callback(&event);
             }