WIP: git behaviour cleanup

 * [x] detect opreation on a git repository
 * [x] create branch specified by `--onto <branch>` if it doesn't exist
 * [ ] when git detected, use `--onto deploy --install --commit` as defaults
 * [ ] make `--onto <branch>` a modifier of `--install` instead of checking out branch
 * [ ] merge unbuilt branch into `--onto <branch>` as separate commit before committing artifacts