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.h b/src/option.h
index 30053cc..7be729a 100644
--- a/src/option.h
+++ b/src/option.h
@@ -127,7 +127,11 @@
 #define ENC_UCSBOM	"ucs-bom"	// check for BOM at start of file
 
 // default value for 'encoding'
-#define ENC_DFLT	"latin1"
+#ifdef MSWIN
+# define ENC_DFLT	"utf-8"
+#else
+# define ENC_DFLT	"latin1"
+#endif
 
 // end-of-line style
 #define EOL_UNKNOWN	-1	// not defined yet