rust-clippy: tell `clippy` we don't need to declare a default here

This is a struct only useful for tests.

rust/hg-core/src/revlog/index.rs

@@ -326,6 +326,7 b' mod tests {'
 
     #[cfg(test)]
     impl IndexEntryBuilder {
+        #[allow(clippy::new_without_default)]
         pub fn new() -> Self {
             Self {
                 is_first: false,