Add option to 'rebase and merge'

Problem to solve

When using "Merge commit with semi-linear history" merge method and my branch cannot be fast-forwarded, then the user has to manually rebase the branch, which is at times cumbersome.

It also encourages users to rebase the remote branch (via the rebase option provided in the MR GUI) which will make it out of sync with their local branch.

Proposal

It should be possible to configure the merge method to automatically attempt a rebase before merging. This is supported by Bitbucket (called "Rebase, merge"). This would make merging a little smoother.