Add rebase with autosquash as merge strategy

Bitbucket Server now supports a multitude of merge strategies. We use the Rebase strategy, and extensively use "git commit --fixup" when resolving issues found in the review, but since there is no way of enabling --autosquash, we are forced to first to a "git rebase --autosquash master" locally, then force-push it to BitBucket, and then finally merge. It would be very convenient if Bitbucket had a merge strategy that did autosquash automatically.