patch 8.2.0518: a terminal falls back to setting $TERM to "xterm"

Problem:    A terminal falls back to setting $TERM to "xterm".
Solution:   Use "xterm-color" if more than 16 colors are supported and
            "xterm-256color" if at least 256 colors are supported.
            (closes #5887)

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    518,
 /**/
     517,
 /**/