rust-changelog: accessing the index

The `Index` object is currently the one providing all DAG related
algorithms, starting with simple ancestors iteration up to more
advanced ones (ranges, common ancestors…).

From pure Rust code, there was no way to access the changelog index for
a given `Repository`, probably because `rhg` does not use any such algorithm
yet.

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

@@ -8,6 +8,7 b' use chrono::{DateTime, FixedOffset, Naiv'
 use itertools::{Either, Itertools};
 
 use crate::errors::HgError;
+use crate::revlog::Index;
 use crate::revlog::Revision;
 use crate::revlog::{Node, NodePrefix};
 use crate::revlog::{Revlog, RevlogEntry, RevlogError};
@@ -81,6 +82,10 b' impl Changelog {'
     ) -> Result<Revision, RevlogError> {
         self.revlog.rev_from_node(node)
     }
+
+    pub fn get_index(&self) -> &Index {
+        &self.revlog.index
+    }
 }
 
 impl Graph for Changelog {