Exclusives bugfix.
Exclusives bugfix.

Builds with no exclusives would sometimes fail due to a bug
in how execution order was planned.

Auditors: benjy

cevaris/commons,pombredanne/commons,brutkin/commons,cevaris/commons,abel-von/commons,VaybhavSharma/commons,nkhuyu/commons,WCCCEDU/twitter-commons,ericzundel/commons,Yasumoto/commons,Yasumoto/commons,WCCCEDU/twitter-commons,pombredanne/commons,jsirois/commons,brutkin/commons,pombredanne/commons,jsirois/commons,nkhuyu/commons,ericzundel/commons,brutkin/commons,abel-von/commons,brutkin/commons,nkhuyu/commons,atollena/commons,Yasumoto/commons,cevaris/commons,pombredanne/commons,cevaris/commons,jsirois/commons,nkhuyu/commons,nkhuyu/commons,Yasumoto/commons,WCCCEDU/twitter-commons,ericzundel/commons,brutkin/commons,abel-von/commons,WCCCEDU/twitter-commons,cevaris/commons,atollena/commons,ericzundel/commons,atollena/commons,jsirois/commons,atollena/commons,abel-von/commons,VaybhavSharma/commons,VaybhavSharma/commons,abel-von/commons,WCCCEDU/twitter-commons,jsirois/commons,VaybhavSharma/commons,VaybhavSharma/commons,pombredanne/commons,Yasumoto/commons,ericzundel/commons,atollena/commons