Stop MR conflict code from blowing up when branches are missing

app/models/merge_request.rb

@@ -299,6 +299,8 @@ class MergeRequest < ActiveRecord::Base
   attr_writer :target_branch_sha, :source_branch_sha
   def source_branch_head
+    return unless source_project
+
     source_branch_ref = @source_branch_sha || source_branch
     source_project.repository.commit(source_branch_ref) if source_branch_ref
   end
@@ -919,6 +921,7 @@ class MergeRequest < ActiveRecord::Base
     return @conflicts_can_be_resolved_in_ui = false unless cannot_be_merged?
     return @conflicts_can_be_resolved_in_ui = false unless has_complete_diff_refs?
+    return @conflicts_can_be_resolved_in_ui = false if branch_missing?
     begin
       # Try to parse each conflict. If the MR's mergeable status hasn't been updated,

lib/gitlab/diff/position_tracer.rb

@@ -16,7 +16,7 @@ module Gitlab
       end
       def trace(old_position)
-        return unless old_diff_refs.complete? && new_diff_refs.complete?
+        return unless old_diff_refs&.complete? && new_diff_refs&.complete?
         return unless old_position.diff_refs == old_diff_refs
         # Suppose we have an MR with source branch `feature` and target branch `master`.

spec/models/merge_request_spec.rb

@@ -1335,6 +1335,13 @@ describe MergeRequest, models: true do
       expect(merge_request.conflicts_can_be_resolved_in_ui?).to be_falsey
     end
+    it 'returns a falsey value when one of the MR branches is missing' do
+      merge_request = create_merge_request('conflict-resolvable')
+      merge_request.project.repository.rm_branch(merge_request.author, 'conflict-resolvable')
+
+      expect(merge_request.conflicts_can_be_resolved_in_ui?).to be_falsey
+    end
+
     it 'returns a falsey value when the MR has a missing ref after a force push' do
       merge_request = create_merge_request('conflict-resolvable')
       allow(merge_request.conflicts).to receive(:merge_index).and_raise(Rugged::OdbError)