Reuse capacity when possible in <BytesMut as Buf>::advance impl (#698)

src/bytes_mut.rs

@@ -1066,6 +1066,14 @@ impl Buf for BytesMut {
 
     #[inline]
     fn advance(&mut self, cnt: usize) {
+        // Advancing by the length is the same as resetting the length to 0,
+        // except this way we get to reuse the full capacity.
+        if cnt == self.remaining() {
+            // SAFETY: Zero is not greater than the capacity.
+            unsafe { self.set_len(0) };
+            return;
+        }
+
         assert!(
             cnt <= self.remaining(),
             "cannot advance past `remaining`: {:?} <= {:?}",