Merge method: Allow multiple merge methods in each repo, and have multiple merge buttons in MRs
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=216125) </details> <!--IssueSummary end--> ### Problem to solve In some repositories, people might want to alternate between merge options like using a merge commit and using a FF merge, with squash or not. The problem is that the current settings only allow one method to be used. ### Intended users This is intended, mostly, for Software Developers, but it might interest any other user that pushes commits to git repositories. ### Proposal I think the radio buttons for the _merge options_ should be replaced with checkboxes, like GitHub does (see image below). Naturally, the internal data structure representing of the selected options would have to be adjusted accordingly. ![image](/uploads/54f20a309cef6644fdae70d7f7b99a12/image.png) The second necessary change is to adapt the `Merge` button in case more than one merge option is allowed. Again, for inspiration, here's how GitHub does it: ![image](/uploads/accf61f5f0db2f1c90feb4aa8855c5a8/image.png)
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