GitalyClient::ConflictsService#conflicts? should return true for conflicts with missing side

lib/gitlab/gitaly_client/conflicts_service.rb

@@ -25,6 +25,11 @@ module Gitlab
 
       def conflicts?
         list_conflict_files.any?
+      rescue GRPC::FailedPrecondition
+        # The server raises this exception when it encounters ConflictSideMissing, which
+        # means a conflict exists but its `theirs` or `ours` data is nil due to a non-existent
+        # file in one of the trees.
+        true
       end
 
       def resolve_conflicts(target_repository, resolution, source_branch, target_branch)

spec/models/repository_spec.rb

@@ -365,12 +365,18 @@ describe Repository do
         it { is_expected.to be_truthy }
       end
 
-      context 'non-mergeable branches' do
+      context 'non-mergeable branches without conflict sides missing' do
         subject { repository.can_be_merged?('bb5206fee213d983da88c47f9cf4cc6caf9c66dc', 'feature') }
 
         it { is_expected.to be_falsey }
       end
 
+      context 'non-mergeable branches with conflict sides missing' do
+        subject { repository.can_be_merged?('conflict-missing-side', 'conflict-start') }
+
+        it { is_expected.to be_falsey }
+      end
+
       context 'non merged branch' do
         subject { repository.merged_to_root_ref?('fix') }