open all folds on disable signify folds

https://github.com/mhinz/vim-signify/blob/master/autoload/sy/fold.vim#L64

I guess for already folded scenarios `zv` makes sense, but if you don't fold anything `zR` backs to the previous view.

Is it possible to remember the previous foldlevel to get back to it?

Read autoload/sy/fold.vim at line 64 and compare the zR and zv behavior described in the issue. Trace what happens when signify is disabled, then determine how the prior foldlevel or view should be retained. Done means disabling signify no longer unexpectedly changes the user's fold state and restores the previous view when appropriate.

Written by the indexing model from the issue text.