arg0: print argv and env

Useful as a replacement utility, when trying to figure out what args and
env is being passed to a CLI hook, like a git hook, or cron job.
cls: clear screen and tmux screen history
gitconfig: enable rerere
git-watch: run an arbitrary command on file change

The set of files that are watched are those under revision control with
git. If you are working on a new file, changes on it won't be noticed
unless you mark your intent to commit it with `git add -N` and restart
git-watch.