fix(filetype): handle .in files with no filename (#30487)

Problem:
fnamemodify with the :r flag will not strip extensions if the filename
starts with a ".". This means that files named ".in" could cause an
infinite loop.

Solution:
Add early return if the filename was not changed

(cherry picked from commit 032e024f8a)

runtime/lua/vim/filetype.lua

@@ -144,6 +144,9 @@ end
 
 local function detect_noext(path, bufnr)
   local root = fn.fnamemodify(path, ':r')
+  if root == path then
+    return
+  end
   return M.match({ buf = bufnr, filename = root })
 end
 
@@ -1258,8 +1261,7 @@ local extension = {
   ['dpkg-new'] = detect_noext,
   ['in'] = function(path, bufnr)
     if vim.fs.basename(path) ~= 'configure.in' then
-      local root = fn.fnamemodify(path, ':r')
-      return M.match({ buf = bufnr, filename = root })
+      return detect_noext(path, bufnr)
     end
   end,
   new = detect_noext,