patch 8.0.1748: CmdlineEnter command uses backslash instead of slash

Problem:    CmdlineEnter command uses backslash instead of slash.
Solution:   Don't treat the character as a file name. (closes #2837)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 2d7956f..1c34e30 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -837,6 +837,8 @@
   au! CmdlineEnter
   au! CmdlineLeave
 
+  let save_shellslash = &shellslash
+  set noshellslash
   au! CmdlineEnter / let g:entered = expand('<afile>')
   au! CmdlineLeave / let g:left = expand('<afile>')
   let g:entered = 0
@@ -849,6 +851,7 @@
   bwipe!
   au! CmdlineEnter
   au! CmdlineLeave
+  let &shellslash = save_shellslash
 endfunc
 
 " Test for BufWritePre autocommand that deletes or unloads the buffer.