Function gh_stack::git::generate_rebase_script

pub fn generate_rebase_script(deps: FlatDep) -> String

or all open pull requests in the graph, generate a series of commands (force-pushes) that will rebase the entire stack. The "PREBASE" variable is a base for the first branch in the stack (essentially a "stop cherry-picking here" marker), and is required because of our squash-merge workflow. TODO: Move this directly into Rust.