Avoid calling underlying methods on non-existing repos

This saves us Rugged/gRPC invocations

app/models/repository.rb

@@ -1031,6 +1031,10 @@ class Repository
     if instance_variable_defined?(ivar)
       instance_variable_get(ivar)
     else
+      # If the repository doesn't exist and a fallback was specified we return
+      # that value inmediately. This saves us Rugged/gRPC invocations.
+      return fallback unless fallback.nil? || exists?
+
       begin
         value =
           if memoize_only
@@ -1040,8 +1044,9 @@ class Repository
           end
         instance_variable_set(ivar, value)
       rescue Rugged::ReferenceError, Gitlab::Git::Repository::NoRepository
-        # if e.g. HEAD or the entire repository doesn't exist we want to
-        # gracefully handle this and not cache anything.
+        # Even if the above `#exists?` check passes these errors might still
+        # occur (for example because of a non-existing HEAD). We want to
+        # gracefully handle this and not cache anything
         fallback
       end
     end

spec/models/repository_spec.rb

@@ -2110,19 +2110,41 @@ describe Repository do
   end
   describe '#cache_method_output', :use_clean_rails_memory_store_caching do
+    let(:fallback) { 10 }
+
     context 'with a non-existing repository' do
-      let(:value) do
-        repository.cache_method_output(:cats, fallback: 10) do
-          raise Rugged::ReferenceError
+      let(:project) { create(:project) } # No repository
+
+      subject do
+        repository.cache_method_output(:cats, fallback: fallback) do
+          repository.cats_call_stub
         end
       end
-      it 'returns a fallback value' do
-        expect(value).to eq(10)
+      it 'returns the fallback value' do
+        expect(subject).to eq(fallback)
+      end
+
+      it 'avoids calling the original method' do
+        expect(repository).not_to receive(:cats_call_stub)
+
+        subject
+      end
+    end
+
+    context 'with a method throwing a non-existing-repository error' do
+      subject do
+        repository.cache_method_output(:cats, fallback: fallback) do
+          raise Gitlab::Git::Repository::NoRepository
+        end
+      end
+
+      it 'returns the fallback value' do
+        expect(subject).to eq(fallback)
       end
       it 'does not cache the data' do
-        value
+        subject
         expect(repository.instance_variable_defined?(:@cats)).to eq(false)
         expect(repository.send(:cache).exist?(:cats)).to eq(false)