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)

runtime/doc/options.txt

@@ -3116,9 +3116,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 	because Vim cannot detect an error, thus the encoding is always
 	accepted.
 	The special value "default" can be used for the encoding from the
-	environment.  This is the default value for 'encoding'.  It is useful
+	environment.  On MS-Windows this is the system encoding.  Otherwise
-	when 'encoding' is set to "utf-8" and your environment uses a
+	this is the default value for 'encoding'.  It is useful when
-	non-latin1 encoding, such as Russian.
+	'encoding' is set to "utf-8" and your environment uses a non-latin1
+	encoding, such as Russian.
 	When 'encoding' is "utf-8" and a file contains an illegal byte
 	sequence it won't be recognized as UTF-8.  You can use the |8g8|
 	command to find the illegal byte sequence.

src/mbyte.c

@@ -4453,8 +4453,13 @@ enc_canonize(char_u *enc)
 
     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);

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2915,
 /**/
     2914,
 /**/