Tweak the ftplugin

Update the ftplugin to behave better. This means use b:did_ftplugin,
toggle cpo as appropriate, and define a b:undo_ftplugin.

ftplugin/snippets.vim

@@ -1,5 +1,13 @@
 " Set some sane defaults for snippet files
 
+if exists('b:did_ftplugin')
+    finish
+endif
+let b:did_ftplugin = 1
+
+let s:save_cpo = &cpo
+set cpo&vim
+
 " Fold by syntax, but open all folds by default
 setlocal foldmethod=syntax
 setlocal foldlevel=99
@@ -14,6 +22,7 @@ setlocal autoindent nosmartindent nocindent
 if exists("loaded_matchit") && !exists("b:match_words")
   let b:match_ignorecase = 0
   let b:match_words = '^snippet\>:^endsnippet\>,^global\>:^endglobal\>,\${:}'
+  let s:set_match_words = 1
 endif
 
 " Add TagBar support
@@ -24,3 +33,15 @@ let g:tagbar_type_snippets = {
             \ ],
             \ 'deffile': expand('<sfile>:p:h:h') . '/ctags/UltiSnips.cnf',
         \ }
+
+" don't unset g:tagbar_type_snippets, it serves no purpose
+let b:undo_ftplugin = "
+            \ setlocal foldmethod< foldlevel< commentstring<
+            \|setlocal expandtab< autoindent< smartindent< cindent<
+            \|if get(s:, 'set_match_words')
+                \|unlet! b:match_ignorecase b:match_words s:set_match_words
+            \|endif
+            \"
+
+let &cpo = s:save_cpo
+unlet s:save_cpo