### Problem to solve This issue is required to work towards enabling merge-ref-head diffs by default: https://gitlab.com/gitlab-org/gitlab/-/issues/198458. When a merge request has conflicts, the merge-ref diff mode does not work. ### Proposal Comparing with head returns base-diff in this case. We just need to implement the logic, which ~~ignores conflicts and continues comparing with head version~~ reverts to base when there are conflicts and informs the user.  Bitbucket, for example, handles this case by displaying the conflicts in the diffs: https://blog.developer.atlassian.com/a-better-pull-request/ <!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION --> *This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.* <!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION -->