As mentioned in an earlier commit, viewoptions is global, so it's
probably best to override it only temporarily for mkview.
(loadview is moved to autoload just for symmetry and in case we need to
do something more complex, like saving/restoring foldmethod, later)
Vimwiki now resets fold{method,text} on BufWinEnter:
8d4cb7f11d/plugin/vimwiki.vim (L177)8d4cb7f11d/plugin/vimwiki.vim (L325)
so we should do that as well otherwise we get overridden.
If the user actually configured vimwiki to use its folding, by setting
g:vimwiki_folding to something else than '' or 'custom', we should not
replace that. It would be nice if we could somehow augment
VimwikiFoldText and leave the rest intact, but we can't, so it's best to
keep off. Using foldmethod=syntax gets extremely laggy with just a few
hundreds of lines, so we must not force it.
Also, we now need to increase foldlevel in tests because the folding is
no longer being overridden in the default vimwiki configuration, so we'd
get closed folds in tests...
Avoids useless reloading of the function. In practice, probably
harmless, but it doesn't belong there, and neither does setting
foldmethod/foldtext, and that is actually harmful (see further commits).
