Supersede pull request (BB-10064)

When a developer makes a pull request, I may want to accept it, but need to make minor tweaks (e.g., fix spelling). It looks nitpicky and isn't a good use of time to force the original submitter, who may be new to Git and to the site, to update their PR.

In such cases, I normally bring their pull request into a local branch, fix it up slightly, then push it (without merging) so that others can look at my version. The problem is that we have to "Decline" every PR that isn't merged verbatim, which looks like rejection to the author even though we accepted it in spirit.

It would be much better if I could make the replacement PR an alternative to the original, so that the original PR could be marked as superseded/accepted with modifications instead of Declined.