Don't attempt swapfiles for man pages

This is because we now use :tag to open a man page, which attempts
to open a swap file for a path under man://...

runtime/autoload/man.vim

@@ -150,6 +150,7 @@ endfunction
 function! s:put_page(page) abort
   setlocal modifiable
   setlocal noreadonly
+  setlocal noswapfile
   silent keepjumps %delete _
   silent put =a:page
   while getline(1) =~# '^\s*$'