This is a huge issue for me both with GitHub and Atlassian Stash.

On both you even cannot see when the branch was started, and you can merge fine as long as there were no merge issues. Then you run gitk and it looks like a spaghetti-horror, with trivial branches being started 200 commits ago.

I don't necessarily need a "rebase&merge" button but at least info about the shared ancestor with master.