Resolve symlinks before running helptags

Helptags seems to fail if some of the path components it receives are
symlinks. So resolve the symlink when calling helptags.

autoload/vundle/installer.vim

@@ -202,7 +202,7 @@ func! s:helptags(rtp) abort
   let doc_path = a:rtp.'/doc/'
   call s:log(':helptags '.doc_path)
   try
-    execute 'helptags ' . doc_path
+    execute 'helptags ' . resolve(doc_path)
   catch
     call s:log("> Error running :helptags ".doc_path)
     return 0