vim-patch:9.0.0771: cannot always tell the difference beween tex and … (#20687)

vim-patch:9.0.0771: cannot always tell the difference beween tex and rexx files

Problem:    Cannot always tell the difference beween tex and rexx files.
Solution:   Recognize tex by a leading backslash. (Martin Tournoij,
            closes vim/vim#11380)
bd053f894b

runtime/lua/vim/filetype/detect.lua

@@ -181,7 +181,7 @@ function M.cls(bufnr)
     return vim.g.filetype_cls
   end
   local line = getlines(bufnr, 1)
-  if line:find('^%%') then
+  if line:find('^[%%\\]') then
     return 'tex'
   elseif line:find('^#') and line:lower():find('rexx') then
     return 'rexx'