Fix 500 error when MR from fork has conflicts but worker has not run

If the ref hasn't been fetched into the target repository yet, this will fail
with a Rugged::ReferencError (assuming we're not using Gitaly). We should handle
this in the same way as a missing ref.

changelogs/unreleased/fix-500-error-when-mr-ref-is-not-yet-fetched.yml

@@ -0,0 +1,6 @@
+---
+title: Fix 500 error when a merge request from a fork has conflicts and has not yet
+  been updated
+merge_request:
+author:
+type: fixed

lib/gitlab/git/conflict/resolver.rb

@@ -23,7 +23,7 @@ module Gitlab
           end
         rescue GRPC::FailedPrecondition => e
           raise Gitlab::Git::Conflict::Resolver::ConflictSideMissing.new(e.message)
-        rescue Rugged::OdbError, GRPC::BadStatus => e
+        rescue Rugged::ReferenceError, Rugged::OdbError, GRPC::BadStatus => e
           raise Gitlab::Git::CommandError.new(e)
         end
 

spec/services/merge_requests/conflicts/list_service_spec.rb

@@ -77,6 +77,14 @@ describe MergeRequests::Conflicts::ListService do
       expect(service.can_be_resolved_in_ui?).to be_falsey
     end
 
+    it 'returns a falsey value when the MR has a missing revision after a force push' do
+      merge_request = create_merge_request('conflict-resolvable')
+      service = conflicts_service(merge_request)
+      allow(merge_request).to receive_message_chain(:target_branch_head, :raw, :id).and_return(Gitlab::Git::BLANK_SHA)
+
+      expect(service.can_be_resolved_in_ui?).to be_falsey
+    end
+
     context 'with gitaly disabled', :skip_gitaly_mock do
       it 'returns a falsey value when the MR has a missing ref after a force push' do
         merge_request = create_merge_request('conflict-resolvable')
@@ -85,6 +93,14 @@ describe MergeRequests::Conflicts::ListService do
 
         expect(service.can_be_resolved_in_ui?).to be_falsey
       end
+
+      it 'returns a falsey value when the MR has a missing revision after a force push' do
+        merge_request = create_merge_request('conflict-resolvable')
+        service = conflicts_service(merge_request)
+        allow(merge_request).to receive_message_chain(:target_branch_head, :raw, :id).and_return(Gitlab::Git::BLANK_SHA)
+
+        expect(service.can_be_resolved_in_ui?).to be_falsey
+      end
     end
   end
 end