In source control systems and tools like tortoises for svn or sourcetree for git, when I am committing a merge, I can usually just left a default comment generated by the tool that is basically a summary of what changes were merged. In visual studio using TFS, there is a comment textbox right above the changes list, but I have to type it all manually. It's not super important but it is a time saver sometimes. Maybe i'm just not looking at the right place.
Try extension Auto Merge
If you need set original comment setup Settings "comment_format" like {{OriginalComment}}