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)
diff --git a/src/testdir/test_filetype.vim b/src/testdir/test_filetype.vim
index 0fb73b7..281642e 100644
--- a/src/testdir/test_filetype.vim
+++ b/src/testdir/test_filetype.vim
@@ -1741,6 +1741,11 @@
   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')
diff --git a/src/version.c b/src/version.c
index e9cacd5..1fc011f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    771,
+/**/
     770,
 /**/
     769,