cp = (oldPrefix, newPrefix, isForce) =>
  mv(oldPrefix, newPrefix, isForce, false)
Clones all tiddler titles that are prefixed with `oldPrefix`
into titles that are instead prefixed with `newPrefix`.

The force option somewhat ensures atomicity.

@param {string} oldPrefix - Moves all tiddlers with this prefix.
@param {string} newPrefix - All tiddlers moved tiddlers will
    receive this new prefix.
@param {boolean} [isForce=false] - If a new title would override
    an existing title, and `force` is not set, then nothing will
    happen and undefined is returned by the function.
@returns {Object<string, string>} - A hashmap that maps the old
    and the new path.