Display merge request conflicts directly in diff

Problem to solve

To avoid displaying the changes that are already on master in the diff, we compare the branch with HEAD of the target branch: #198458 (closed). In order to do that, we artificially merge the target branch to the source branch and compare this merge-branch to the target branch. However, when the target branch conflicts with the source branch, we're unable to merge it, so currently we display master (base) version in this case.

User experience goal

Display master (HEAD) version with explicit conflicts in the diff, something like BitBucket solves this problem:

Proposal

One of the ways to reach this goal is to either modify Gitaly::UserMergeToRefRequest request or create a new one which force merge the conflicts into the diff and explicitly list us the files which contain conflicts. After that, we need to correctly highlight the conflict-block. It is a raw idea though and should be thoroughly investigated. A good links from Bitbucket development: https://blog.developer.atlassian.com/a-better-pull-request/

• Merge source branch to target with adding the conflicts files as-is (with conflict markers)
• Highlight the conflicts section properly in the diffs

The motivation behind the changes: https://www.youtube.com/watch?v=GFXIFA4ZuZw&feature=youtu.be

Feature flag

The functionality is behind display_merge_conflicts_in_diff feature flag

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