Allow resolving conflicts with non-ASCII chars

What does this MR do?

Actually allow resolving conflicts in file that are UTF-8, instead of just saying that we do.

Are there points in the code the reviewer needs to double check?

My logic at https://gitlab.com/gitlab-org/gitlab-ce/issues/26214#note_25334508 (and repeated in the commit message) may be faulty. Please think seriously about this.

Why was this MR needed?

The docs were wrong! We only allowed resolving conflicts in ASCII files.

Screenshots (if relevant)

Using this test MR: https://gitlab.com/davidmbroin/test-mr-confilcts/merge_requests/2

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/26214.