patch 9.0.1550: in cmdline window S-Tab does not select previous completion

Problem:    In cmdline window S-Tab does not select previous completion.
            (Maxim Kim)
Solution:   Add a mappint for S-Tab. (closes #12116)
diff --git a/src/ex_getln.c b/src/ex_getln.c
index 21447ff..595286a 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -4486,8 +4486,12 @@
     {
 	if (p_wc == TAB)
 	{
+	    // Make Tab start command-line completion: CTRL-X CTRL-V
 	    add_map((char_u *)"<buffer> <Tab> <C-X><C-V>", MODE_INSERT, TRUE);
 	    add_map((char_u *)"<buffer> <Tab> a<C-X><C-V>", MODE_NORMAL, TRUE);
+
+	    // Make S-Tab work like CTRL-P in command-line completion
+	    add_map((char_u *)"<buffer> <S-Tab> <C-P>", MODE_INSERT, TRUE);
 	}
 	set_option_value_give_err((char_u *)"ft",
 					       0L, (char_u *)"vim", OPT_LOCAL);