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 #11380)

runtime/autoload/dist/ft.vim

@@ -145,7 +145,7 @@ export def FTcls()
     return
   endif
 
-  if getline(1) =~ '^%'
+  if getline(1) =~ '^\v%(\%|\\)'
     setf tex
   elseif getline(1)[0] == '#' && getline(1) =~ 'rexx'
     setf rexx

src/testdir/test_filetype.vim

@@ -1741,6 +1741,11 @@ func Test_cls_file()
   call assert_equal('tex', &filetype)
   bwipe!
 
+  call writefile(['\NeedsTeXFormat{LaTeX2e}'], 'Xfile.cls')
+  split Xfile.cls
+  call assert_equal('tex', &filetype)
+  bwipe!
+
   " Rexx
 
   call writefile(['# rexx'], 'Xfile.cls')

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    771,
 /**/
     770,
 /**/