vimPlugins: automatically commit redirect updates

- When redirections are detected, rather than printing instructions,
update.py now creates two commits -- one with the plugin updates and
another with redirected plugin names updated to their new repos.
- Added a --allow-dirty flag so that one can run ./update.py --commit
with a dirty nixpkgs repository, which is necessary for development.
I wouldn't mind removing this before merging if it's not in our flag
budget but it's necessary scaffolding for now.