bin: add git-churn from garybernhardt's dotfiles

Available here: https://github.com/garybernhardt/dotfiles/blob/f0c0ff92209e5aed4fa3ef6faf056eb9944a8f12/bin/git-churn
bin: add gn to summarize unified diffs

Available here https://github.com/garybernhardt/dotfiles/blob/f0c0ff92209e5aed4fa3ef6faf056eb9944a8f12/bin/gn
bin: add git-divergence to summarize changes between remotes

Available here https://github.com/garybernhardt/dotfiles/blob/f0c0ff92209e5aed4fa3ef6faf056eb9944a8f12/bin/git-divergence