patch 7.4.1012

Problem:    Vim overwrites the value of $PYTHONHOME.
Solution:   Do not set $PYTHONHOME if it is already set. (Kazuki Sakamoto,
            closes #500)

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1012,
 /**/
     1011,
 /**/