Add robots.txt for incoming.debian.org.

Files on incoming.debian.org only exist for a short while so it makes no
sense to have them indexed.

config/debian/apache.conf-incoming

@@ -13,6 +13,8 @@
     Order deny,allow
   </Directory>
 
+  Alias /robots.txt /srv/incoming.debian.org/robots.txt
+
   # buildd direct access -- buildds only
   Alias /debian /srv/ftp.debian.org/mirror
   Alias /debian-buildd /srv/incoming.debian.org/debian-buildd

config/debian/robots.txt-incoming

+User-agent: *
+Disallow: /