git: Only save dirty files during staging (#41047)

Closes #40581

Release Notes:

- git: No longer save clean files when staging (to avoid triggering
unnecessary rebuilds in external file watchers like vite)

crates/language/src/buffer.rs

@@ -2008,7 +2008,7 @@ impl Buffer {
         self.end_transaction(cx)
     }
 
-    fn has_unsaved_edits(&self) -> bool {
+    pub fn has_unsaved_edits(&self) -> bool {
         let (last_version, has_unsaved_edits) = self.has_unsaved_edits.take();
 
         if last_version == self.version {