A new repository for a story (a group of small tasks)? That sounds like an overkill. Repositories carry some operational overhead like users, policies, pipelines and security access, and others ...
I agree about the overhead. How about, developer can create her/his own personal forked repo and she/he can create pull request to it's parent repo? This way we can get rid of overhead.
But then wouldn't they end up just using branches to prevent merge commit hell?
By which point they may as well just work on a branch on the main repo...
Plus, that approach could get kind of messy when groups of developers are working on stories.
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