merge: conflicts information might be missing in case of multiple line conflicts

In case of the merge of 3 lines, if in next version all lines changed, and in current version the first and last lines changed, then one conflict is detected, and the associated chunk for the current version only concerns the first line.
So we basically lose the information about the last line conflict.

The expected result here is not clear, it can be:
1. to return 2 conflicts, for the first line and for the last line: the chunk of next version should only concern the first line then
2. to return 1 conflict but then to return in both case all 3 lines.