Feature summary (what you would like to be able to do and where):
When using Special:MergeHistory, I propose that an entry be placed in the page's history that states that a history merge took place. On both the old page and the new page.
This is already done for page moving and page protection. Whatever mechanism that uses, I'd suggest copying it. A null edit maybe?
Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Benefits (why should this be implemented?):