rust-revlog: a trait for the revlog index

As explained in the doc comment, this is the minimum needed
for our immediate concern, which is to implement a nodemap
in Rust.

The trait will be later implemented in `hg-cpython` by the
index Python object implemented in C, thanks to exposition
of the corresponding functions as a capsule.

The `None` return cases in `node()` match what the `index_node()`
C function does.

Differential Revision: https://phab.mercurial-scm.org/D7789

rust/hg-core/src/revlog.rs

@@ -40,3 +40,17 b' pub enum GraphError {'
     ParentOutOfRange(Revision),
     WorkingDirectoryUnsupported,
 }
+
+/// The Mercurial Revlog Index
+///
+/// This is currently limited to the minimal interface that is needed for
+/// the [`nodemap`](nodemap/index.html) module
+pub trait RevlogIndex {
+    /// Total number of Revisions referenced in this index
+    fn len(&self) -> usize;
+
+    /// Return a reference to the Node or `None` if rev is out of bounds
+    ///
+    /// `NULL_REVISION` is not considered to be out of bounds.
+    fn node(&self, rev: Revision) -> Option<&Node>;
+}