Improve messaging on UI Rebase button, avoid unintentional rebases
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A customer\[[Internal ticket](https://gitlab.zendesk.com/agent/tickets/589178)\] noted that the option to select `Rebase` on the UI may mislead developers to rebase unnecessarily.
**Suggestion**: Make the UI Rebase button for merge requests with (semi-)linear or Fast-forward merge methods optional if there are no conflicts.
<p>Alternatively, change the message on the UI to indicate it as a suggestion rather than the sure next step. </p>
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