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/runtime/doc/options.txt b/runtime/doc/options.txt
index b2b79cb..d3adbd0 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -3116,9 +3116,10 @@
 	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
-	when 'encoding' is set to "utf-8" and your environment uses a
-	non-latin1 encoding, such as Russian.
+	environment.  On MS-Windows this is the system encoding.  Otherwise
+	this is the default value for 'encoding'.  It is useful when
+	'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.
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);
diff --git a/src/version.c b/src/version.c
index 56ce534..8584778 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2915,
+/**/
     2914,
 /**/
     2913,