Allow missing side when listing conflicts on diffs

What does this MR do?

In gitaly!3648 (merged), support for listing conflicts even when there's a missing side has been added to ListConflictFiles RPC. This only happens when allow_tree_conflicts is set to true in the RPC request.

This allows gitlab-rails to use that parameter when displaying MR diffs with conflicts. The allow_tree_conflicts will be set to true only when display_merge_conflicts_in_diff is enabled. Otherwise, it'll be set to false.

Does this MR meet the acceptance criteria?

Conformity

• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Related to #333687 (closed)