graph: extract function that reverses graph edges

ReverseGraphIterator will be inlined. It doesn't make sense the iterator yields
Item = Result<..> whereas all possible errors are confined by constructor.

lib/src/graph.rs

@@ -85,6 +85,23 @@ where
     pub fn new(
         input: impl Iterator<Item = Result<GraphNode<N>, RevsetEvaluationError>>,
     ) -> Result<Self, RevsetEvaluationError> {
+        let items = reverse_graph(input)?;
+        Ok(Self { items })
+    }
+}
+
+impl<N> Iterator for ReverseGraphIterator<N> {
+    type Item = Result<GraphNode<N>, RevsetEvaluationError>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.items.pop().map(Ok)
+    }
+}
+
+/// Creates new graph in which edges are reversed.
+fn reverse_graph<N: Clone + Eq + Hash, E>(
+    input: impl Iterator<Item = Result<GraphNode<N>, E>>,
+) -> Result<Vec<GraphNode<N>>, E> {
     let mut entries = vec![];
     let mut reverse_edges: HashMap<N, Vec<GraphEdge<N>>> = HashMap::new();
     for item in input {
@@ -103,16 +120,7 @@ where
         let edges = reverse_edges.get(&node).cloned().unwrap_or_default();
         items.push((node, edges));
     }
-        Ok(Self { items })
-    }
-}
-
-impl<N> Iterator for ReverseGraphIterator<N> {
-    type Item = Result<GraphNode<N>, RevsetEvaluationError>;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.items.pop().map(Ok)
-    }
+    Ok(items)
 }
 
 /// Graph iterator adapter to group topological branches.