serve: Fix repeated error message when HTML config is invalid

This fixes an issue where `mdbook serve` would repeatedly show an error
message every second if the `output.html` config had a problem. The
issue is that `set_roots` was doing more work than I realized. It is not
necessary to call it every time in the main polling loop, since the
roots only change when the book configuration changes.

The solution is to just reset the roots whenever the book config
changes.

Fixes https://github.com/rust-lang/mdBook/issues/2946

src/cmd/watch/poller.rs

@@ -41,7 +41,6 @@ pub fn rebuild_on_change(
 
     loop {
         std::thread::sleep(Duration::new(1, 0));
-        watcher.set_roots(&book);
         let start = Instant::now();
         let paths = watcher.scan();
         let elapsed = start.elapsed().as_secs_f64();
@@ -67,6 +66,7 @@ pub fn rebuild_on_change(
                         post_build();
                     }
                     book = b;
+                    watcher.set_roots(&book);
                 }
                 Err(e) => error!("failed to load book config: {e:?}"),
             }