Fix unfinished merge by Merge Train process

This causes merge trains to be stuck.

The fix here is to handle Gitlab::Git::CommandError
for Branches::DeleteService

app/services/branches/delete_service.rb

@@ -26,7 +26,7 @@ module Branches
           message: 'Failed to remove branch',
           http_status: 400)
       end
-    rescue Gitlab::Git::PreReceiveError => ex
+    rescue Gitlab::Git::PreReceiveError, Gitlab::Git::CommandError => ex
       ServiceResponse.error(message: ex.message, http_status: 400)
     end
 

changelogs/unreleased/217908-handle-git-errors.yml

+---
+title: Fix unfinished merge by Merge Train process
+merge_request: 41106
+author:
+type: fixed

spec/services/branches/delete_service_spec.rb

@@ -37,6 +37,21 @@ RSpec.describe Branches::DeleteService do
       end
 
       it_behaves_like 'a deleted branch', 'feature'
+
+      context 'when Gitlab::Git::CommandError is raised' do
+        before do
+          allow(repository).to receive(:rm_branch) do
+            raise Gitlab::Git::CommandError.new('Could not update patch')
+          end
+        end
+
+        it 'handles and returns error' do
+          result = service.execute('feature')
+
+          expect(result.status).to eq(:error)
+          expect(result.message).to eq('Could not update patch')
+        end
+      end
     end
 
     context 'when user does not have access to push to repository' do