Handle removed source projects in MR CI commits

When calling MergeRequest#ci_commit the code would previously raise an
error if the source project no longer existed (e.g. because the user
removed their fork).

See #3599 for more information.

app/models/merge_request.rb

@@ -476,7 +476,7 @@ class MergeRequest < ActiveRecord::Base
   end
   def ci_commit
-    if last_commit
+    if last_commit and source_project
       source_project.ci_commit(last_commit.id)
     end
   end

spec/models/merge_request_spec.rb

@@ -193,4 +193,29 @@ describe MergeRequest do
   it_behaves_like 'a Taskable' do
     subject { create :merge_request, :simple }
   end
+
+  describe '#ci_commit' do
+    describe 'when the source project exists' do
+      it 'returns the latest commit' do
+        commit    = double(:commit, id: '123abc')
+        ci_commit = double(:ci_commit)
+
+        allow(subject).to receive(:last_commit).and_return(commit)
+
+        expect(subject.source_project).to receive(:ci_commit).
+          with('123abc').
+          and_return(ci_commit)
+
+        expect(subject.ci_commit).to eq(ci_commit)
+      end
+    end
+
+    describe 'when the source project does not exist' do
+      it 'returns nil' do
+        allow(subject).to receive(:source_project).and_return(nil)
+
+        expect(subject.ci_commit).to be_nil
+      end
+    end
+  end
 end