patch 9.1.0539: Not enough tests for what v9.1.0535 fixed

Problem:  Not enough tests for what v9.1.0535 fixed
Solution: Add another test for ex-mode

This comes from: https://groups.google.com/g/vim_dev/c/F5-tDqoafz8/m/GqKF-uQsLD0J

related: #15120

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_ex_mode.vim

@@ -346,4 +346,11 @@ func Test_backslash_multiline()
 \
 .', "xt")
   call assert_equal(["", "enum"], getline(1, 2))
+endfunc
+
+" Test using backslash in ex-mode after patch 9.1.0535
+func Test_backslash_multiline2()
+  new
+  call feedkeys('Qa
+X \\
 Y

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    539,
 /**/
     538,
 /**/