"Purge recent activity" function (BB-9385)

I could image a scenario where someone accidentally puts sensitive information into a commit message, and would want to eliminate it from the history. One can "rebase" a branch, which could permanently delete that commit from the repository history, but the "Recent activity" display on the "Overview" tab would still show the commit message from when it was originally pushed.

It would be handy to have a button in the settings called "Purge recent activity" which would remove any such sensitive commit messages.