Prevent caching of buffer commentstring (#515)

Co-authored-by: Yuxin Wu <ppwwyyxx@users.noreply.github.com>

autoload/nerdcommenter.vim

@@ -461,10 +461,6 @@ endif
 " This function is responsible for setting up buffer scoped variables for the
 " current buffer.
 function! nerdcommenter#SetUp() abort
-    if exists('b:NERDCommenterDelims')
-        return
-    endif
-
     let filetype = &filetype
 
     "for compound filetypes, if we don't know how to handle the full filetype
@@ -496,8 +492,8 @@ function! nerdcommenter#SetUp() abort
         " if g:NERD_<filetype>_alt_style is defined, use the alternate style
         let b:NERDCommenterFirstInit = getbufvar(1,'NERDCommenterFirstInit')
         if exists('g:NERDAltDelims_'.filetype) && eval('g:NERDAltDelims_'.filetype) && !b:NERDCommenterFirstInit
-            call nerdcommenter#SwitchToAlternativeDelimiters(0)
             let b:NERDCommenterFirstInit = 1
+            call nerdcommenter#SwitchToAlternativeDelimiters(0)
         endif
     else
         let b:NERDCommenterDelims = s:CreateDelimMapFromCms()