Require tangles.group.previous to be at least size 1

[Powersource]

In the current tanglePrune code from tribes1 it is able to prune until the previous array is empty. Is that a good idea or likely to break tangle logic?

ssb-tribes2/lib/tangle-prune.js

Lines 23 to 28 in ff6905b

	while (
	content.tangles[tangle].previous.length &&
	getLength(content) > maxSize
	) {
	content.tangles[tangle].previous.pop()
	}

[mixmix]

Nice catch. I think it should prune down to one at lowest

[Powersource]

The new logic has fixed this

ssb-tribes2/lib/prune-publish.js

Lines 5 to 22 in 206138e

	module.exports = function prunePublish(ssb, content, keys, cb) {
	const group = content.tangles.group
	ssb.db.create({ keys, content, encryptionFormat: 'box2' }, (err, msg) => {
	if (err) {
	if (group.previous.length > 1) {
	const halfLength = Math.ceil(group.previous.length / 2)
	group.previous = group.previous.slice(0, halfLength)
	return prunePublish(ssb, content, keys, cb)
	} else {
	return cb(err)
	}
	}
	cb(null, msg)
	})
	}