zed: Mark restored buffers as conflicted if file changed on disk between store & restore (#15207)

Previously, we've only marked restored buffers as dirty. This PR changes
that behavior in case the buffer has been associated with a file and
that file has changed on disk since the last time Zed stored its
contents.

Example timeline:

1. User edits file in Zed, buffer is dirty
2. User quites Zed with `cmd-q`
3. User changes file on disk: `echo foobar >> file.txt` or `git checkout
file.txt`
4. User starts Zed
5. File/buffer are now marked as having a conflict (yellow icon)

Release Notes:

- Unsaved files that are restored when Zed starts are now marked as
having a conflict if they have been changed on disk since the last time
they were stored.

Demo:



https://github.com/user-attachments/assets/6098b485-b325-49b7-b694-fd2fc60cce64

crates/fs/src/fs.rs

@@ -821,6 +821,11 @@ impl FakeFs {
         })
     }
 
+    pub fn set_next_mtime(&self, next_mtime: SystemTime) {
+        let mut state = self.state.lock();
+        state.next_mtime = next_mtime;
+    }
+
     pub async fn insert_file(&self, path: impl AsRef<Path>, content: Vec<u8>) {
         self.write_file_internal(path, content).unwrap()
     }