rust-index: don't use mutable borrow for head-diff computation stable

rust-index: don't use mutable borrow for head-diff computation

It does not needs to mutate the index.

This is one of the two suspects of RuntimeError being thrown during push.

--- a/rust/hg-cpython/src/revlog.rs	Thu Feb 29 14:13:21 2024 -0800
+++ b/rust/hg-cpython/src/revlog.rs	Tue Mar 05 15:07:04 2024 +0100
@@ -854,7 +854,7 @@
     ) -> PyResult<PyObject> {
         let begin = begin.extract::<BaseRevision>(py)?;
         let end = end.extract::<BaseRevision>(py)?;
-        let index = &mut *self.index(py).borrow_mut();
+        let index = &*self.index(py).borrow();
         let begin =
             Self::check_revision(index, UncheckedRevision(begin - 1), py)?;
         let end = Self::check_revision(index, UncheckedRevision(end - 1), py)?;