ignore: add debug log message when opening gitignore file

I'm not sure why it took me this long to add this debug message, but
it's quite useful in determining where ignore rules are coming from.

crates/ignore/src/gitignore.rs

@@ -390,6 +390,7 @@ impl GitignoreBuilder {
             Err(err) => return Some(Error::Io(err).with_path(path)),
             Ok(file) => file,
         };
+        log::debug!("opened gitignore file: {}", path.display());
         let rdr = BufReader::new(file);
         let mut errs = PartialErrorBuilder::default();
         for (i, line) in rdr.lines().enumerate() {