We could simply count the commits

app/models/merge_request.rb

@@ -899,7 +899,8 @@ class MergeRequest < ActiveRecord::Base
   def compute_diverged_commits_count
     return 0 unless source_branch_sha && target_branch_sha
-    Gitlab::Git::Commit.between(target_project.repository.raw_repository, source_branch_sha, target_branch_sha).size
+    target_project.repository
+      .count_commits_between(source_branch_sha, target_branch_sha)
   end
   private :compute_diverged_commits_count

changelogs/unreleased/use-count_commits-directly.yml

+---
+title: Improve the performance for counting commits
+merge_request: 15628
+author:
+type: performance

lib/gitlab/git/repository.rb

@@ -505,7 +505,7 @@ module Gitlab
       # Counts the amount of commits between `from` and `to`.
       def count_commits_between(from, to)
-        Commit.between(self, from, to).size
+        count_commits(ref: "#{from}..#{to}")
       end
       # Returns the SHA of the most recent common ancestor of +from+ and +to+