Allow setting merge strategy per branch

Problem to solve

Allow the merge strategy to be set at the branch level.

Further details

For example, if I have three protected branches (master, stage, production), I'd like to the ability to set the following merge strategies:

• master: Fast-forward
• stage: merge commit
• production: fast-forward

Documentation

https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html

What does success look like, and how can we measure that?

Ability to set the merge strategy at the branch level

Links / references

Customer ticket: https://gitlab.zendesk.com/agent/tickets/116976 (internal use)