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)
diff --git a/src/version.c b/src/version.c
index 4c07cd2..f420990 100644
--- a/src/version.c
+++ b/src/version.c
@@ -739,6 +739,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    518,
+/**/
     517,
 /**/
     516,