ftplugin: Avoid changing global viewoptions

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)

autoload/taskwiki.vim

@@ -1,6 +1,18 @@
 if exists('g:loaded_taskwiki_auto') | finish | endif
 let g:loaded_taskwiki_auto = 1
 
+function! taskwiki#MkView() abort
+  let viewoptions = &viewoptions
+  set viewoptions-=options
+  mkview
+  let &viewoptions = viewoptions
+endfunction
+
+function! taskwiki#LoadView() abort
+  silent! loadview
+  silent! doautocmd SessionLoadPost
+endfunction
+
 function! taskwiki#FoldInit() abort
   " Unless vimwiki is configured to use its folding, set our own
   if &foldtext !~? 'VimwikiFold'

ftplugin/vimwiki/taskwiki.vim

@@ -41,10 +41,8 @@ augroup taskwiki
     autocmd BufWrite <buffer> TaskWikiBufferSave
     " Save and load the view to preserve folding, if desired
     if !exists('g:taskwiki_dont_preserve_folds')
-      setlocal viewoptions-=options
-      autocmd BufWinLeave <buffer> mkview
-      autocmd BufWinEnter <buffer> silent! loadview
-      autocmd BufWinEnter <buffer> silent! doautocmd SessionLoadPost
+      autocmd BufWinLeave <buffer> call taskwiki#MkView()
+      autocmd BufWinEnter <buffer> call taskwiki#LoadView()
     endif
     " Reset cache when switching buffers
     execute "autocmd BufEnter <buffer> :" . g:taskwiki_py . "cache.load_current().reset()"