perf: don't compute locals on buffer updates

Instead we lazily evaluate them on request.
This allow two things :
    * better performances
    * being sure the locas are up to date

lua/nvim-treesitter.lua

@@ -9,12 +9,6 @@ local M = {}
 -- this is the main interface through the plugin
 function M.setup(lang)
   if parsers.has_parser(lang) then
-    if locals.is_supported(lang) then
-      print("Locals setup for", lang)
-      api.nvim_command(string.format([[
-      autocmd NvimTreesitter FileType %s lua vim.api.nvim_buf_attach(0, true, {on_lines=require'nvim-treesitter.locals'.on_lines})
-      ]], lang))
-    end
   end
 end
 

lua/nvim-treesitter/locals.lua

@@ -10,18 +10,14 @@ local M = {
   locals={}
 }
 
-function M.is_supported(lang)
-  return queries.get_query(lang, "locals") ~= nil
-end
-
 function M.collect_locals(bufnr)
   local ft = api.nvim_buf_get_option(bufnr, "ft")
-
   if not ft then return end
 
   local query = queries.get_query(ft, 'locals')
-  local parser = parsers.get_parser(bufnr, ft)
+  if not query then return end
 
+  local parser = parsers.get_parser(bufnr, ft)
   if not parser then return end
 
   local root = parser:parse():root()
@@ -36,12 +32,18 @@ function M.collect_locals(bufnr)
   return locals
 end
 
-function M.on_lines(_, buf, _, firstline, lastline, new_lastline)
-  M.locals[buf] = M.collect_locals(buf)
+local function update_cached_locals(bufnr, changed_tick)
+  M.locals[bufnr] = {tick=changed_tick, cache=( M.collect_locals(bufnr) or {} )}
 end
 
 function M.get_locals(bufnr)
-  return M.locals[bufnr or api.nvim_get_current_buf()] or {}
+  local bufnr = bufnr or api.nvim_get_current_buf()
+  local cached_local = M.locals[bufnr]
+  if not cached_local or api.nvim_buf_get_changedtick(bufnr) < cached_local.tick then
+    update_cached_locals(bufnr,api.nvim_buf_get_changedtick(bufnr))
+  end
+
+  return M.locals[bufnr].cache
 end
 
 function M.get_definitions(bufnr)