Mark conflicting lines in the the unified diff of the changes column.
It seems to be the case that a "conflicting line" equals a line where in both version - the one you edited and the one someone else edited - something was changed in comparison to the original version.
In the current version of the TwoColumnConflict extension this is already visible when "mine" and "your" changes appear in one paragraph. I guess this must not necessarily be marked with a special hint.
Furthermore its not quite clear what should be achieved when introducing an extra hint to theses lines. So closed for now.