Make sure, that vtysh_cmd.c is deterministically generated.

--- a/vtysh/extract.pl.in	2017-03-10 13:55:06.000000000 +0100
+++ b/vtysh/extract.pl.in	2017-06-24 00:51:56.460000000 +0200
@@ -214,7 +214,7 @@
 }
 
 # Output DEFSH
-foreach (keys %live) {
+foreach (sort keys %live) {
     my ($proto);
     my ($key);
     $key = $live{$_};
@@ -229,7 +229,7 @@
 {
 EOF
 
-foreach (keys %odefun) {
+foreach (sort keys %odefun) {
     my ($node, $str) = (split (/,/));
     $cmd = $ocmd{$_};
     $cmd =~ s/_cmd/_cmd_vtysh/;
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* doc/contributing.texi (The Perfect Setup): Document this.
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