function getMissingDeps(backend, heads = []) {
  return backendState(backend).getMissingDeps(heads)
}
Returns the hashes of any missing dependencies, i.e. where we have applied a
change that has a dependency on a change we have not seen.

If the argument `heads` is given (an array of hexadecimal strings representing
hashes as returned by `getHeads()`), this function also ensures that all of
those hashes resolve to either a change that has been applied to the document,
or that has been enqueued for later application once missing dependencies have
arrived. Any missing heads hashes are included in the returned array.