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/option.c b/src/option.c
index 7c78375..6ca424e 100644
--- a/src/option.c
+++ b/src/option.c
@@ -430,14 +430,21 @@
 #  endif
 # endif
 
+# ifdef MSWIN
+    // MS-Windows has builtin support for conversion to and from Unicode, using
+    // "utf-8" for 'encoding' should work best for most users.
+    p = vim_strsave((char_u *)ENC_DFLT);
+# else
     // enc_locale() will try to find the encoding of the current locale.
+    // This works best for properly configured systems, old and new.
     p = enc_locale();
+# endif
     if (p != NULL)
     {
 	char_u *save_enc;
 
 	// Try setting 'encoding' and check if the value is valid.
-	// If not, go back to the default "latin1".
+	// If not, go back to the default encoding.
 	save_enc = p_enc;
 	p_enc = p;
 	if (STRCMP(p_enc, "gb18030") == 0)