Clean up git behaviour

 * [x] detect git repository
 * [x] make `--onto <branch>` a modifier of `--install` instead of checking out branch
 * [x] make `--onto BRANCH` default to `"deploy"`
 * [x] when git detected, use `--install --commit` as default operation (implicit `--onto deploy`)
 * [x] when committing, create branch if it doesn't exist
 * [x] merge HEAD into deploy branch as separate commit before committing artifacts
 * [x] skip merge if destination branch already up to date
 * [x] don't commit build artifacts if they are identical to the target branch's content
 * [x] update existing tests to match new behaviour
 * [x] add tests to verify new behaviour
I guess these tests would have to be converted back to JS. I'm in favour of leaving them as is until then, so that we can benefit from the simplicity as long as possible.