patch 8.1.0050: $VIM_TERMINAL is also set when not in a terminal window

Problem:    $VIM_TERMINAL is also set when not in a terminal window.
Solution:   Pass a flag to indicate whether the job runs in a terminal.

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    50,
 /**/
     49,
 /**/