Fix MergeRequest#source_sha when there is no diff

`MergeRequest#source_sha` is expected to return the sha of the source
branch last commit.

But when a open Merge Request has no diff (e.g. all commits have already
been merged to the target branch), `merge_request.source_sha`
incorrectly returns `nil`.

This was un-noticed before – but now that !2217 has been merged,
it makes `Gitlab::Git::Commit.between` raise an
"Unexpected nil argument" exception.

This fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available.

CHANGELOG

@@ -33,6 +33,7 @@ v 8.6.0 (unreleased)
   - Fix bug where Bitbucket `closed` issues were imported as `opened` (Iuri de Silvio)
   - Don't show Issues/MRs from archived projects in Groups view
   - Fix wrong "iid of max iid" in Issuable sidebar for some merged MRs
+  - Fix empty source_sha on Merge Request when there is no diff (Pierre de La Morinerie)
   - Increase the notes polling timeout over time (Roberto Dip)
   - Add shortcut to toggle markdown preview (Florent Baldino)
   - Show labels in dashboard and group milestone views

app/models/merge_request.rb

@@ -520,7 +520,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def source_sha
-    last_commit.try(:sha)
+    last_commit.try(:sha) || source_tip.try(:sha)
+  end
+
+  def source_tip
+    source_branch && source_project.repository.commit(source_branch)
   end
 
   def fetch_ref

spec/factories/merge_requests.rb

@@ -51,6 +51,11 @@ FactoryGirl.define do
     trait :with_diffs do
     end
 
+    trait :without_diffs do
+      source_branch "improve/awesome"
+      target_branch "master"
+    end
+
     trait :conflict do
       source_branch "feature_conflict"
       target_branch "feature"

spec/models/merge_request_spec.rb

@@ -96,6 +96,13 @@ describe MergeRequest, models: true do
       end
     end
 
+    context 'without diffs' do
+      subject { create(:merge_request, :without_diffs) }
+      it 'returns the sha of the source branch last commit' do
+        expect(subject.source_sha).to eq(last_branch_commit.sha)
+      end
+    end
+
     context 'when the merge request is being created' do
       subject { build(:merge_request, source_branch: nil, compare_commits: []) }
       it 'returns nil' do