zsh: add -r to parents

zsh: expand tags lazily

make mq play nicely with the branch cache - if no patches are applied, don't do anything different - if the cache includes valid data from one of the patch revisions, use the cache, but don't save anything new - if the cache has data from before the patch revisions only, save what the list of branches would be without the patch revisions

Split branchtags into two additional functions. This makes it easier to override only parts of the cache saving process.