add --rebase flag to mr merge

[maxice8]

Describe the feature or problem you'd like to solve
At alpine/aports we need all commits to be rebased against master before merging so there are no merge commits and we have a nice linear history (see screenshot below)

It would be very convenient to have a flag --rebase or preferably a way to automatically rebase whenever a merge request needs it due to the situation above.

Propose a Solution
Have a flag --rebase that does the equivalent of glab mr rebase before doing the merge itself

Additional context
This helps with a race condition where a rebase might end before and the attempt to merge after another person has pushed.

1. Start rebase
2. Rebase finishes
3. Someone pushes to trunk or merge another MR
4. Attempt to merge
5. Has to rebase again because of 3

having --rebase while not completely eliminating the race condition (unlike GitHub does with the rebase method for merging a pull request) greatly reduces the possibility of it happening.

[clemsbot]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[clemsbot]

This issue has been automatically marked as stale because it has not had recent activity. We haven't had the time to address it yet, but we want to keep it open. This message is just a reminder for us to help triage issues.

[profclems]

This is resolved by #695