Speed up generation of commit stats by using Rugged native methods

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

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

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

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