Improve code by using wrapping_neg

Author: GuillaumeGomez

Diff for: library/core/src/fmt/num.rs

@@ -219,17 +219,13 @@ macro_rules! impl_Display {
         #[stable(feature = "rust1", since = "1.0.0")]
         impl fmt::Display for $signed {
             fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                let is_nonnegative = *self >= 0;
-
-                if !is_nonnegative {
+                if *self < 0 {
                     #[cfg(not(feature = "optimize_for_size"))]
                     {
-                        // convert the negative num to positive by summing 1 to its 2s complement
                         return (!self as $unsigned).wrapping_add(1)._fmt(false, f);
                     }
                     #[cfg(feature = "optimize_for_size")]
                     {
-                        // convert the negative num to positive by summing 1 to its 2s complement
                         return $gen_name((!self.$conv_fn()).wrapping_add(1), false, f);
                     }
                 }