dot_git_dir.go

func dot_git_dir(dir string) string {
	git_dir := path.Join(dir, ".git")
	_, dir_error := os.Stat(git_dir)
	if dir_error != nil {
		if git_dir == "/.git" {
			log.Fatal("Cannot find a .git directory")
		}
		return dot_git_dir(path.Dir(dir))
	} else {
		return git_dir
	}
}

It looks like you're dealing with file paths here, so the correct package to use would be path/filepath, not path. That way it's more readable and has a better chance of working on all 3 OSes.

Using path instead of path/filepath feels like seeing someone use text/template instead of html/template to generate HTML... It may work for simple cases, but it's better to use the correct package and be safe.

Idiomatic golang uses the early return pattern, which means you can eliminate the else part of your code. See:
https://golang.org/doc/effective_go.html#if