Handle error when fetching ref for MR with deleted source branch

If the ref doesn't exist, and the source branch is deleted, we can't get it back
easily. Previously, we ignored this error by shelling out, so replicate that
behaviour.

changelogs/unreleased/38319-nomethoderror-undefined-method-sha-for-nil-nilclass.yml

+---
+title: Fix 500 error on merged merge requests when GitLab is restored from a backup
+merge_request:
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -947,7 +947,11 @@ module Gitlab
         if start_repository == self
           yield commit(start_branch_name)
         else
-          sha = start_repository.commit(start_branch_name).sha
+          start_commit = start_repository.commit(start_branch_name)
+
+          return yield nil unless start_commit
+
+          sha = start_commit.sha
           if branch_commit = commit(sha)
             yield branch_commit
@@ -976,8 +980,9 @@ module Gitlab
         with_repo_branch_commit(source_repository, source_branch) do |commit|
           if commit
             write_ref(local_ref, commit.sha)
+            true
           else
-            raise Rugged::ReferenceError, 'source repository is empty'
+            false
           end
         end
       end

spec/lib/gitlab/git/repository_spec.rb

@@ -1332,6 +1332,84 @@ describe Gitlab::Git::Repository, seed_helper: true do
     end
   end
+  describe '#with_repo_branch_commit' do
+    context 'when comparing with the same repository' do
+      let(:start_repository) { repository }
+
+      context 'when the branch exists' do
+        let(:start_branch_name) { 'master' }
+
+        it 'yields the commit' do
+          expect { |b| repository.with_repo_branch_commit(start_repository, start_branch_name, &b) }
+            .to yield_with_args(an_instance_of(Gitlab::Git::Commit))
+        end
+      end
+
+      context 'when the branch does not exist' do
+        let(:start_branch_name) { 'definitely-not-master' }
+
+        it 'yields nil' do
+          expect { |b| repository.with_repo_branch_commit(start_repository, start_branch_name, &b) }
+            .to yield_with_args(nil)
+        end
+      end
+    end
+
+    context 'when comparing with another repository' do
+      let(:start_repository) { Gitlab::Git::Repository.new('default', TEST_MUTABLE_REPO_PATH, '') }
+
+      context 'when the branch exists' do
+        let(:start_branch_name) { 'master' }
+
+        it 'yields the commit' do
+          expect { |b| repository.with_repo_branch_commit(start_repository, start_branch_name, &b) }
+            .to yield_with_args(an_instance_of(Gitlab::Git::Commit))
+        end
+      end
+
+      context 'when the branch does not exist' do
+        let(:start_branch_name) { 'definitely-not-master' }
+
+        it 'yields nil' do
+          expect { |b| repository.with_repo_branch_commit(start_repository, start_branch_name, &b) }
+            .to yield_with_args(nil)
+        end
+      end
+    end
+  end
+
+  describe '#fetch_source_branch' do
+    let(:local_ref) { 'refs/merge-requests/1/head' }
+
+    context 'when the branch exists' do
+      let(:source_branch) { 'master' }
+
+      it 'writes the ref' do
+        expect(repository).to receive(:write_ref).with(local_ref, /\h{40}/)
+
+        repository.fetch_source_branch(repository, source_branch, local_ref)
+      end
+
+      it 'returns true' do
+        expect(repository.fetch_source_branch(repository, source_branch, local_ref)).to eq(true)
+      end
+    end
+
+    context 'when the branch does not exist' do
+      let(:source_branch) { 'definitely-not-master' }
+
+      it 'does not write the ref' do
+        expect(repository).not_to receive(:write_ref)
+
+        repository.fetch_source_branch(repository, source_branch, local_ref)
+      end
+
+      it 'returns false' do
+        expect(repository.fetch_source_branch(repository, source_branch, local_ref)).to eq(false)
+      end
+    end
+  end
+
   def create_remote_branch(repository, remote_name, branch_name, source_branch_name)
     source_branch = repository.branches.find { |branch| branch.name == source_branch_name }
     rugged = repository.rugged