rewrite: extract compute_commits_heads function

This will be shared between `move_commits` and the new
`duplicate_commits` function to be added.

lib/src/rewrite.rs

@@ -587,21 +587,7 @@ pub fn move_commits(
     let new_children_parents: HashMap<_, _> = if !new_children.is_empty() {
         // Compute the heads of the target set, which will be used as the parents of
         // `new_children`.
-        let mut target_heads: HashSet<CommitId> = HashSet::new();
-        for commit in connected_target_commits.iter().rev() {
-            target_heads.insert(commit.id().clone());
-            for old_parent in commit.parent_ids() {
-                target_heads.remove(old_parent);
-            }
-        }
-        let target_heads = connected_target_commits
-            .iter()
-            .rev()
-            .filter(|commit| {
-                target_heads.contains(commit.id()) && target_commit_ids.contains(commit.id())
-            })
-            .map(|commit| commit.id().clone())
-            .collect_vec();
+        let target_heads = compute_commits_heads(&target_commit_ids, &connected_target_commits);
 
         new_children
             .iter()
@@ -824,6 +810,32 @@ fn compute_internal_parents_within(
     internal_parents
 }
 
+/// Computes the heads of commits in the target set, given the list of
+/// `target_commit_ids` and a connected graph of commits.
+///
+/// `connected_target_commits` should be in reverse topological order (children
+/// before parents).
+fn compute_commits_heads(
+    target_commit_ids: &HashSet<CommitId>,
+    connected_target_commits: &[Commit],
+) -> Vec<CommitId> {
+    let mut target_head_ids: HashSet<CommitId> = HashSet::new();
+    for commit in connected_target_commits.iter().rev() {
+        target_head_ids.insert(commit.id().clone());
+        for old_parent in commit.parent_ids() {
+            target_head_ids.remove(old_parent);
+        }
+    }
+    connected_target_commits
+        .iter()
+        .rev()
+        .filter(|commit| {
+            target_head_ids.contains(commit.id()) && target_commit_ids.contains(commit.id())
+        })
+        .map(|commit| commit.id().clone())
+        .collect_vec()
+}
+
 pub struct CommitToSquash {
     pub commit: Commit,
     pub selected_tree: MergedTree,