patch 8.2.2915: MS-Windows: when using "default" for encoding utf-8 is used

Problem:    MS-Windows: when using "default" for encoding utf-8 is used.
Solution:   Use the system encoding. (Ken Takata, closes #8300)
diff --git a/src/mbyte.c b/src/mbyte.c
index f3fdc32..697e585 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -4453,8 +4453,13 @@
 
     if (STRCMP(enc, "default") == 0)
     {
+#ifdef MSWIN
+	// Use the system encoding, the default is always utf-8.
+	r = enc_locale();
+#else
 	// Use the default encoding as it's found by set_init_1().
 	r = get_encoding_default();
+#endif
 	if (r == NULL)
 	    r = (char_u *)ENC_DFLT;
 	return vim_strsave(r);