repo: move test_id_index() into a tests module

This is the usual convention (to save on compilation time when not
running tests).

lib/src/repo.rs

@@ -1237,6 +1237,10 @@ impl IdIndex {
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+
     #[test]
     fn test_id_index() {
         let mut id_index = IdIndex::new();
@@ -1260,3 +1264,4 @@ fn test_id_index() {
         // If it were there, the length would be 1.
         assert_eq!(id_index.shortest_unique_prefix_len(b"c"), 1);
     }
+}