rust: Make private the `index` field of the `Revlog` struct

rust: Make private the `index` field of the `Revlog` struct

To replace the previous use of this field from another module, add a
`node_from_rev` method. This is the same method that already existed on
`Changelog`.

Differential Revision: https://phab.mercurial-scm.org/D11414

--- a/rust/hg-core/src/revlog/changelog.rs	Mon Jul 26 10:26:45 2021 +0200
+++ b/rust/hg-core/src/revlog/changelog.rs	Tue Sep 14 18:07:11 2021 +0200
@@ -36,7 +36,7 @@
     }
 
     pub fn node_from_rev(&self, rev: Revision) -> Option<&Node> {
-        Some(self.revlog.index.get_entry(rev)?.hash())
+        self.revlog.node_from_rev(rev)
     }
 }
 

--- a/rust/hg-core/src/revlog/revlog.rs	Mon Jul 26 10:26:45 2021 +0200
+++ b/rust/hg-core/src/revlog/revlog.rs	Tue Sep 14 18:07:11 2021 +0200
@@ -18,7 +18,7 @@
 use crate::errors::HgError;
 use crate::repo::Repo;
 use crate::revlog::Revision;
-use crate::NULL_REVISION;
+use crate::{Node, NULL_REVISION};
 
 #[derive(derive_more::From)]
 pub enum RevlogError {
@@ -51,7 +51,7 @@
     /// When index and data are not interleaved: bytes of the revlog index.
     /// When index and data are interleaved: bytes of the revlog index and
     /// data.
-    pub(crate) index: Index,
+    index: Index,
     /// When index and data are not interleaved: bytes of the revlog data
     data_bytes: Option<Box<dyn Deref<Target = [u8]> + Send>>,
     /// When present on disk: the persistent nodemap for this revlog
@@ -119,6 +119,11 @@
         self.index.is_empty()
     }
 
+    /// Returns the node ID for the given revision number, if it exists in this revlog
+    pub fn node_from_rev(&self, rev: Revision) -> Option<&Node> {
+        Some(self.index.get_entry(rev)?.hash())
+    }
+
     /// Return the full data associated to a node.
     #[timed]
     pub fn get_node_rev(