Interesting concept, but merging individual files seems like a moot point. When you branch off you usually branch off with the intention to add or edit specific files, while someone else works on another section that targets different files.
If you are thinking of adding in extra functionality on one branch that directly interfaces with scripts stored on the branch someone else is working on you would usually branch off and implement those scripts after the original branches have been merged.
It is an interesting point, however, I feel like doing this would ultimately result in merge conflicts