Dedicated method for counting commits between refs

gitlab_git 8.1 adds the ability to count the amount of commits between
two references without having to allocate anything but regular
Rugged::Commit objects. This in turn speeds up the process of counting
the number of commits a branch is ahead/behind by about 3.5x.

Gemfile

@@ -50,7 +50,7 @@ gem "browser", '~> 1.0.0'
 
 # Extracting information from a git repository
 # Provide access to Gitlab::Git library
-gem "gitlab_git", '~> 8.0.0'
+gem "gitlab_git", '~> 8.1'
 
 # LDAP Auth
 # GitLab fork with several improvements to original library. For full list of changes

Gemfile.lock

@@ -357,7 +357,7 @@ GEM
       posix-spawn (~> 0.3)
     gitlab_emoji (0.2.0)
       gemojione (~> 2.1)
-    gitlab_git (8.0.0)
+    gitlab_git (8.1.0)
       activesupport (~> 4.0)
       charlock_holmes (~> 0.7.3)
       github-linguist (~> 4.7.0)
@@ -933,7 +933,7 @@ DEPENDENCIES
   github-markup (~> 1.3.1)
   gitlab-flowdock-git-hook (~> 1.0.1)
   gitlab_emoji (~> 0.2.0)
-  gitlab_git (~> 8.0.0)
+  gitlab_git (~> 8.1)
   gitlab_meta (= 7.0)
   gitlab_omniauth-ldap (~> 1.2.1)
   gollum-lib (~> 4.1.0)

app/models/repository.rb

@@ -184,8 +184,11 @@ class Repository
     cache.fetch(:"diverging_commit_counts_#{branch.name}") do
       # Rugged seems to throw a `ReferenceError` when given branch_names rather
       # than SHA-1 hashes
-      number_commits_behind = commits_between(branch.target, root_ref_hash).size
-      number_commits_ahead = commits_between(root_ref_hash, branch.target).size
+      number_commits_behind = raw_repository.
+        count_commits_between(branch.target, root_ref_hash)
+
+      number_commits_ahead = raw_repository.
+        count_commits_between(root_ref_hash, branch.target)
 
       { behind: number_commits_behind, ahead: number_commits_ahead }
     end