Set COMMIT_DIFFS_PER_PAGE=20 for CommitController

This should improve the memory usage the overall user experience as
diffs may be chunky and its rendering uses significant amount of memory
and cpu resources

app/controllers/projects/commit_controller.rb

@@ -24,7 +24,7 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   BRANCH_SEARCH_LIMIT = 1000
-  COMMIT_DIFFS_PER_PAGE = 75
+  COMMIT_DIFFS_PER_PAGE = 20
 
   feature_category :source_code_management
 

spec/helpers/commits_helper_spec.rb

@@ -168,15 +168,15 @@ RSpec.describe CommitsHelper do
         let(:page) { 1 }
 
         it "has 20 diffs" do
-          expect(subject.size).to eq(75)
+          expect(subject.size).to eq(20)
         end
       end
 
-      context "page 2" do
-        let(:page) { 2 }
+      context "page 5" do
+        let(:page) { 5 }
 
-        it "has the remaining 10 diffs" do
-          expect(subject.size).to eq(10)
+        it "has the remaining 5 out of 85 diffs" do
+          expect(subject.size).to eq(5)
         end
       end
     end