patch 8.2.4844: <C-S-I> is simplified to <S-Tab>

Problem:    <C-S-I> is simplified to <S-Tab>.
Solution:   Do not simplify CTRL if there is also SHIFT. (closes #10313)
diff --git a/src/getchar.c b/src/getchar.c
index 2b1bbde..6d8697c 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -1634,7 +1634,7 @@
 {
     int c = c_arg;
 
-    if (*modifiers & MOD_MASK_CTRL)
+    if ((*modifiers & MOD_MASK_CTRL) && !(*modifiers & MOD_MASK_SHIFT))
     {
 	if ((c >= '`' && c <= 0x7f) || (c >= '@' && c <= '_'))
 	{