Handle non-UTF-8 conflicts gracefully
## What does this MR do?
If a conflict file isn't in a UTF-8-compatible encoding, we can't resolve it in the UI.
## What are the relevant issue numbers?
Closes#21247.
See merge request !5961
These can't be resolved in the UI because if they aren't in a UTF-8
compatible encoding, they can't be rendered as JSON. Even if they could,
we would be implicitly changing the file encoding anyway, which seems
like a bad idea.
An MR can only be resolved in the UI if:
- It has conflicts.
- It has valid diff_refs (in other words, it supports new diff notes).
- It has no conflicts with one side missing.
- It has no conflicts in binary files.
- It has no conflicts in files too large to display.
- It has no conflicts containing invalid conflict markers.
- Add match line header to expected result for `File#sections`.
- Lowercase CSS colours.
- Remove unused `diff_refs` keyword argument.
- Rename `parent` -> `parent_file`, to be more explicit.
- Skip an iteration when highlighting.
