patch 8.2.4798: t_8u option was reset even when set by the user

Problem:    t_8u option was reset even when set by the user.
Solution:   Only reset t_8u when using the default value. (closes #10239)

src/term.c

@@ -4826,7 +4826,9 @@ handle_version_response(int first, int *arg, int argc, char_u *tp)
 	// This may cause some flicker.  Alternative would be to set "t_8u"
 	// here if the terminal is expected to support it, but that might
 	// conflict with what was set in the .vimrc.
-	if (term_props[TPR_UNDERLINE_RGB].tpr_status != TPR_YES && *T_8U != NUL)
+	if (term_props[TPR_UNDERLINE_RGB].tpr_status != TPR_YES
+			&& *T_8U != NUL
+			&& !option_was_set((char_u *)"t_8u"))
 	{
 	    set_string_option_direct((char_u *)"t_8u", -1, (char_u *)"",
 								  OPT_FREE, 0);

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4798,
 /**/
     4797,
 /**/