The idea is terrific but focus is all wrong. Writers don't need branches. They don't code features in. Hell, most developers don't do stupid updates-on-branch-then-merge-merge-suffer-merge-curse-merge commit accounting.

Writers usually write alone, what they need is: A (desktop) tool with basic WYSIWYG markup (paragraphs, bold/italic, that sort of thing) with version control, visual diffs, export into common formats. Maybe a pull request or two.