rust-utils: remove buggy assertion

While this assertion had good intentions, it broke existing behavior with a
nasty panic.

Differential Revision: https://phab.mercurial-scm.org/D6651

rust/hg-core/src/utils.rs

@@ -18,8 +18,7 b' pub fn replace_slice<T>(buf: &mut [T], f'
 where
     T: Clone + PartialEq,
 {
-    assert_eq!(from.len(), to.len());
-    if buf.len() < from.len() {
+    if buf.len() < from.len() || from.len() != to.len() {
         return;
     }
     for i in 0..=buf.len() - from.len() {