debian-specific: process only one upload at a time

Otherwise if one accepts multiple uploads in `dak process-new`, but
only edits the `COMMENTS` file for one `.changes` file one would end
up accepting an upload into a public suite.  This would be a problem
for uploads fixing embargoed issues.

docs/debian-specific.rst

@@ -91,7 +91,7 @@ NEW processing
 
     cronoff
     CHANGES=FILENAME.changes
-    dak process-new
+    dak process-new ${CHANGES}
     cd /srv/security-master.debian.org/queue/new/COMMENTS
     echo $'NOTOK\n\nMoving back to unchecked' > "REJECT.${CHANGES%.changes}"
     rm "ACCEPT.${CHANGES%.changes}"