Do not return negative numbers from exceeded_size

Negative numbers are not needed from this method and may be confusing.

ee/lib/ee/gitlab/repository_size_checker.rb

@@ -17,9 +17,10 @@ module EE
       override :exceeded_size
       # @param change_size [int] in bytes
       def exceeded_size(change_size = 0)
-        exceeded_size = super
-        exceeded_size -= remaining_additional_purchased_storage if additional_repo_storage_available?
-        exceeded_size
+        size = super
+        size -= remaining_additional_purchased_storage if additional_repo_storage_available?
+
+        [size, 0].max
       end
 
       private

ee/spec/lib/ee/gitlab/repository_size_checker_spec.rb

@@ -144,8 +144,8 @@ RSpec.describe Gitlab::RepositorySizeChecker do
           context 'when current size + total repository size excess is below the limit (additional purchase storage not used)' do
             let(:current_size) { limit - 1 }
 
-            it 'returns a negative number' do
-              expect(subject.exceeded_size).to eq(-1.megabyte)
+            it 'returns zero' do
+              expect(subject.exceeded_size).to eq(0)
             end
           end
 

lib/gitlab/repository_size_checker.rb

@@ -37,7 +37,9 @@ module Gitlab
 
     # @param change_size [int] in bytes
     def exceeded_size(change_size = 0)
-      current_size + change_size - limit
+      size = current_size + change_size - limit
+
+      [size, 0].max
     end
 
     def error_message

spec/support/shared_examples/lib/gitlab/repository_size_checker_shared_examples.rb

@@ -21,8 +21,8 @@ RSpec.shared_examples 'checker size exceeded' do
     context 'when current size is below the limit' do
       let(:current_size) { limit - 1 }
 
-      it 'returns a negative number' do
-        expect(subject.exceeded_size).to eq(-1.megabyte)
+      it 'returns zero' do
+        expect(subject.exceeded_size).to eq(0)
       end
     end
 
@@ -67,7 +67,7 @@ RSpec.shared_examples 'checker size exceeded' do
       let(:current_size) { limit - 2 }
 
       it 'returns zero' do
-        expect(subject.exceeded_size(change_size)).to eq(-1.megabyte)
+        expect(subject.exceeded_size(change_size)).to eq(0)
       end
     end
   end