Issue Summary

When comparing branches through the UI, it is possible to choose "Fast forward" only when attempting to sync a fork with its upstream parent. However, Git itself doesn't allow merging diverging branches with fast-forward.

Steps to Reproduce

1. Navigate to a fork of a repository;
2. Navigate to the 'Commits' section;
3. Click the 'Compare' button in the top right;
4. Choose the branches you want to sync between the fork and parent;
5. Click 'Compare;
6. Click 'Merge' in the top right;
7. In the merge menu, change the merge method to 'Fast Forward Only' and click 'Merge'.

Expected Results

• An error should appear, stating that syncing forks with FF is not possible

Actual Results

• The merge request initiates and fails after a while with a 500 in the network tab

Workaround

• Use a different merge strategy to sync the fork