Use memoization for commits on diffs

The Gitaly CommitService is being hammered by n + 1 calls, mostly when
finding commits. This leads to this gRPC being turned of on production:
https://gitlab.com/gitlab-org/gitaly/issues/514#note_48991378

Hunting down where it came from, most of them were due to
MergeRequest#show. To prove this, I set a script to request the
MergeRequest#show page 50 times. The GDK was being scraped by
Prometheus, where we have metrics on controller#action and their Gitaly
calls performed. On both occations I've restarted the full GDK so all
caches had to be rebuild.

Current master, 806a68a81f1baee, needed 435 requests
After this commit, 154 requests

2 files changed, 16 insertions, 1 deletions

diff --git a/spec/lib/gitlab/utils/strong_memoize_spec.rb b/spec/lib/gitlab/utils/strong_memoize_spec.rb
index 4a104ab6d97..474532dd82b 100644
--- a/spec/lib/gitlab/utils/strong_memoize_spec.rb
+++ b/spec/lib/gitlab/utils/strong_memoize_spec.rb
@@ -49,4 +49,17 @@ describe Gitlab::Utils::StrongMemoize do
       end
     end
   end
+
+  describe '#clear_memoization' do
+    let(:value) { 'mepmep' }
+
+    it 'removes the instance variable' do
+      object.method_name
+
+      object.clear_memoization(:method_name)
+
+      expect(object.instance_variable_defined?(:@method_name)).to be(false)
+    end
+  end
 end
+
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 30a5a3bbff7..6b98d013ded 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -124,6 +124,7 @@ describe MergeRequest do
     context 'when the target branch does not exist' do
       before do
         project.repository.rm_branch(subject.author, subject.target_branch)
+        subject.clear_memoized_shas
       end
 
       it 'returns nil' do
@@ -733,7 +734,7 @@ describe MergeRequest do
 
       before do
         project.repository.raw_repository.delete_branch(subject.target_branch)
-        subject.reload
+        subject.clear_memoized_shas
       end
 
       it 'does not crash' do
@@ -1468,6 +1469,7 @@ describe MergeRequest do
     context 'when the target branch does not exist' do
       before do
         subject.project.repository.rm_branch(subject.author, subject.target_branch)
+        subject.clear_memoized_shas
       end
 
       it 'returns nil' do