Return conflict_type property in diffs_batch.json endpoint

What does this MR do?

In order for frontend to display appropriate message when a file has a conflict, they need to know which type of conflict has occurred.

This adds the conflict_type property which can be the following:

• both_modified
• modified_source_removed_target
• modified_target_removed_source
• renamed_same_file
• removed_source_renamed_target
• removed_target_renamed_source
• both_added

No changelog entry as this endpoint is not part of the REST API.

Does this MR meet the acceptance criteria?

Conformity

• 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.
• This change is backwards compatible across updates, or this does not apply.

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 #281171 (closed)