An independent blockchain would work better, but the main problem is that codebases are BIG. But a DAG like git suits the blockchain perfectly. I've actually considered the idea a few times but have been deterred by the amount of storage required... But a @utopian.io type of system could definitely work together with repositories. I don't think that Steem could run it right now, but it would be easy to implement.
I would love to see people being able to upvote issues, pull requests, etc. Maybe even tokens could be given out whenever a pull request happens (with Sybil defense obviously)