`hub init` doesn't imply `hub create`. I've written wrong documentation
in 0a45aa8d

Fixes #1810

Fixes #862

Now the optional `directory` argument to `git init` is detected by
looking for the first "word" in arguments that isn't a value for one of
`--template`, `--shared`, or `--separate-git-dir` params.

This fixes #790.

- Use a hash to lookup for commands.
- Add more tests for the command and runner execution.