patch 8.2.2912: MS-Windows: most users expect using Unicode

Problem:    MS-Windows: most users expect using Unicode.
Solution:   Default 'encoding' to utf-8 on MS-Windows. (Ken Takata,
            closes #3907)
diff --git a/src/mbyte.c b/src/mbyte.c
index e3d64ba..f3fdc32 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -4456,7 +4456,7 @@
 	// Use the default encoding as it's found by set_init_1().
 	r = get_encoding_default();
 	if (r == NULL)
-	    r = (char_u *)"latin1";
+	    r = (char_u *)ENC_DFLT;
 	return vim_strsave(r);
     }