Speed up generation of commit stats by using Rugged native methodssh-optimize-commit-stats

The previous implementation iterated across the entire patch set
to determine the number of lines added, deleted, and changed. Rugged
has a native method `Rugged::Diff#stat` that does this already,
which appears to be a little faster and require less RAM than doing
this ourselves.

Improves performance in #41524

3 files changed, 12 insertions, 7 deletions

diff --git a/changelogs/unreleased/sh-optimize-commit-stats.yml b/changelogs/unreleased/sh-optimize-commit-stats.yml
new file mode 100644
index 00000000000..8c1be1252fb
--- /dev/null
+++ b/changelogs/unreleased/sh-optimize-commit-stats.yml
@@ -0,0 +1,5 @@
+---
+title: Speed up generation of commit stats by using Rugged native methods
+merge_request:
+author:
+type: performance
diff --git a/lib/gitlab/git/commit_stats.rb b/lib/gitlab/git/commit_stats.rb
index 6bf49a0af18..8463b1eb794 100644
--- a/lib/gitlab/git/commit_stats.rb
+++ b/lib/gitlab/git/commit_stats.rb
@@ -34,13 +34,8 @@ module Gitlab
 
       def rugged_stats(commit)
         diff = commit.rugged_diff_from_parent
-
-        diff.each_patch do |p|
-          # TODO: Use the new Rugged convenience methods when they're released
-          @additions += p.stat[0]
-          @deletions += p.stat[1]
-          @total += p.changes
-        end
+        _files_changed, @additions, @deletions = diff.stat
+        @total = @additions + @deletions
       end
     end
   end
diff --git a/spec/lib/gitlab/git/commit_spec.rb b/spec/lib/gitlab/git/commit_spec.rb
index 5ed639543e0..6d35734d306 100644
--- a/spec/lib/gitlab/git/commit_spec.rb
+++ b/spec/lib/gitlab/git/commit_spec.rb
@@ -428,6 +428,11 @@ describe Gitlab::Git::Commit, seed_helper: true do
       subject { super().deletions }
       it { is_expected.to eq(6) }
     end
+
+    describe '#total' do
+      subject { super().total }
+      it { is_expected.to eq(17) }
+    end
   end
 
   describe '#stats with gitaly on' do