core: add a 'trace' message

This message will emit the binary detection mechanism being used for
each file.

This does not noticeably increases the number of log messages, as the
'trace' level is already used for emitting messages for every file
searched.

This trace message was added in the course of investigating #1838.

crates/core/search.rs

@@ -325,9 +325,10 @@ impl<W: WriteColor> SearchWorker<W> {
         } else {
             self.config.binary_implicit.clone()
         };
-        self.searcher.set_binary_detection(bin);
-
         let path = subject.path();
+        log::trace!("{}: binary detection: {:?}", path.display(), bin);
+
+        self.searcher.set_binary_detection(bin);
         if subject.is_stdin() {
             self.search_reader(path, io::stdin().lock())
         } else if self.should_preprocess(path) {