index: add methods that provides commit/change_id_length

We could add Layout struct holding these parameters, but I don't think that's
needed just for two parameters.

lib/src/default_index/mutable.rs

@@ -72,8 +72,8 @@ impl MutableIndexSegment {
 
     pub(super) fn incremental(parent_file: Arc<ReadonlyIndexSegment>) -> Self {
         let num_parent_commits = parent_file.as_composite().num_commits();
-        let commit_id_length = parent_file.commit_id_length;
-        let change_id_length = parent_file.change_id_length;
+        let commit_id_length = parent_file.commit_id_length();
+        let change_id_length = parent_file.change_id_length();
         Self {
             parent_file: Some(parent_file),
             num_parent_commits,

lib/src/default_index/readonly.rs

@@ -126,8 +126,8 @@ pub(super) struct ReadonlyIndexSegment {
     parent_file: Option<Arc<ReadonlyIndexSegment>>,
     num_parent_commits: u32,
     name: String,
-    pub(super) commit_id_length: usize,
-    pub(super) change_id_length: usize,
+    commit_id_length: usize,
+    change_id_length: usize,
     commit_graph_entry_size: usize,
     commit_lookup_entry_size: usize,
     // Number of commits not counting the parent file
@@ -215,6 +215,14 @@ impl ReadonlyIndexSegment {
         &self.name
     }
 
+    pub(super) fn commit_id_length(&self) -> usize {
+        self.commit_id_length
+    }
+
+    pub(super) fn change_id_length(&self) -> usize {
+        self.change_id_length
+    }
+
     fn graph_entry(&self, local_pos: u32) -> CommitGraphEntry {
         let offset = (local_pos as usize) * self.commit_graph_entry_size;
         CommitGraphEntry {