patch 8.1.0585: undo test may fail on MS-Windows

Problem:    Undo test may fail on MS-Windows.
Solution:   Also handle lower case drive letters.

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    585,
 /**/
     584,
 /**/