auto merge of #4045 : mttr/servo/mach_update, r=jdm
auto merge of #4045 : mttr/servo/mach_update, r=jdm

Example usage:

``` sh
$ ./mach update-cargo -p rust-xml
.
    Updating git repository `https://github.com/netvl/rust-xml`
ports/cef
    Updating git repository `https://github.com/netvl/rust-xml`
ports/android/glut_app
    Updating git repository `https://github.com/netvl/rust-xml`
```

GreenRecycleBin/servo,nnethercote/servo,runarberg/servo,huonw/servo,pyfisch/servo,meh/servo,mt2d2/servo,ConnorGBrewster/servo,canaltinova/servo,Adenilson/prototype-viewing-distance,ruud-v-a/servo,jimberlage/servo,szeged/servo,dvberkel/servo,jimberlage/servo,rixrix/servo,steveklabnik/servo,peterjoel/servo,GreenRecycleBin/servo,kindersung/servo,GyrosOfWar/servo,samfoo/servo,pyfisch/servo,brendandahl/servo,codemac/servo,tschneidereit/servo,evilpie/servo,pyfisch/servo,nnethercote/servo,avadacatavra/servo,Shraddha512/servo,wpgallih/servo,fiji-flo/servo,peterjoel/servo,rnestler/servo,dagnir/servo,RenaudParis/servo,anthgur/servo,dvberkel/servo,mbrubeck/servo,dmarcos/servo,ryancanhelpyou/servo,dsandeephegde/servo,brendandahl/servo,emilio/servo,chotchki/servo,peterjoel/servo,GyrosOfWar/servo,s142857/servo,emilio/servo,akosel/servo,g-k/servo,ruud-v-a/servo,saneyuki/servo,caldwell/servo,runarberg/servo,rixrix/servo,paulrouget/servo,mbrubeck/servo,rentongzhang/servo,meh/servo,chotchki/servo,walac/servo,jimberlage/servo,GreenRecycleBin/servo,tempbottle/servo,evilpie/servo,fiji-flo/servo,mt2d2/servo,paulrouget/servo,avadacatavra/servo,g-k/servo,codemac/servo,canaltinova/servo,upsuper/servo,sadmansk/servo,saratang/servo,AnthonyBroadCrawford/servo,sadmansk/servo,wartman4404/servo,emilio/servo,mbrubeck/servo,jlegendary/servo,saneyuki/servo,wartman4404/servo,nick-thompson/servo,ConnorGBrewster/servo,hyowon/servo,rnestler/servo,vks/servo,luniv/servo,mdibaiee/servo,deokjinkim/servo,A-deLuna/servo,GreenRecycleBin/servo,avadacatavra/servo,youprofit/servo,echochamber/servo,s142857/servo,deokjinkim/servo,canaltinova/servo,zhangjunlei26/servo,notriddle/servo,evilpie/servo,mattnenterprise/servo,jgraham/servo,mbrubeck/servo,bjwbell/servo,karlito40/servo,notriddle/servo,rnestler/servo,jimberlage/servo,CJ8664/servo,luniv/servo,jlegendary/servo,WriterOfAlicrow/servo,notriddle/servo,cbrewster/servo,jdramani/servo,bfrohs/servo,tafia/servo,hyowon/servo,akosel/servo,walac/servo,nnethercote/servo,dsandeephegde/servo,nick-thompson/servo,tempbottle/servo,vks/servo,dvberkel/servo,dagnir/servo,rnestler/servo,boghison/servo,dhananjay92/servo,KiChjang/servo,emilio/servo,jimberlage/servo,Shraddha512/servo,karlito40/servo,tafia/servo,GyrosOfWar/servo,thiagopnts/servo,tempbottle/servo,shrenikgala/servo,nerith/servo,jlegendary/servo,samfoo/servo,codemac/servo,tschneidereit/servo,splav/servo,tempbottle/servo,rixrix/servo,aidanhs/servo,nerith/servo,paulrouget/servo,RenaudParis/servo,samfoo/servo,dati91/servo,mattnenterprise/servo,nnethercote/servo,GreenRecycleBin/servo,thiagopnts/servo,Shraddha512/servo,wartman4404/servo,AnthonyBroadCrawford/servo,indykish/servo,dsandeephegde/servo,mattnenterprise/servo,youprofit/servo,larsbergstrom/servo,rnestler/servo,fiji-flo/servo,pyecs/servo,froydnj/servo,runarberg/servo,upsuper/servo,steveklabnik/servo,nerith/servo,michaelwu/servo,s142857/servo,snf/servo,upsuper/servo,meh/servo,dsandeephegde/servo,jgraham/servo,pyfisch/servo,canaltinova/servo,srbhklkrn/SERVOENGINE,runarberg/servo,splav/servo,WriterOfAlicrow/servo,Adenilson/prototype-viewing-distance,dagnir/servo,brendandahl/servo,boghison/servo,tafia/servo,paulrouget/servo,upsuper/servo,SimonSapin/servo,tempbottle/servo,dhananjay92/servo,KiChjang/servo,GreenRecycleBin/servo,srbhklkrn/SERVOENGINE,nick-thompson/servo,jimberlage/servo,aidanhs/servo,bfrohs/servo,dvberkel/servo,meh/servo,pyfisch/servo,aidanhs/servo,emilio/servo,dmarcos/servo,walac/servo,nerith/servo,dmarcos/servo,emilio/servo,pyecs/servo,zentner-kyle/servo,aidanhs/servo,saratang/servo,evilpie/servo,szeged/servo,eddyb/servo,srbhklkrn/SERVOENGINE,szeged/servo,tafia/servo,mbrubeck/servo,samfoo/servo,deokjinkim/servo,peterjoel/servo,peterjoel/servo,anthgur/servo,vks/servo,bjwbell/servo,evilpie/servo,ConnorGBrewster/servo,rixrix/servo,thiagopnts/servo,CJ8664/servo,mt2d2/servo,saneyuki/servo,mbrubeck/servo,saneyuki/servo,luniv/servo,karlito40/servo,caldwell/servo,nnethercote/servo,echochamber/servo,boghison/servo,youprofit/servo,huonw/servo,DominoTree/servo,WriterOfAlicrow/servo,bfrohs/servo,sadmansk/servo,jgraham/servo,thiagopnts/servo,pyecs/servo,boghison/servo,DominoTree/servo,nrc/servo,dmarcos/servo,chotchki/servo,kindersung/servo,s142857/servo,froydnj/servo,indykish/servo,saratang/servo,dati91/servo,indykish/servo,pyfisch/servo,ryancanhelpyou/servo,szeged/servo,larsbergstrom/servo,larsbergstrom/servo,saneyuki/servo,nnethercote/servo,zentner-kyle/servo,runarberg/servo,shrenikgala/servo,Adenilson/prototype-viewing-distance,dagnir/servo,KiChjang/servo,cbrewster/servo,caldwell/servo,GreenRecycleBin/servo,dati91/servo,DominoTree/servo,splav/servo,pyfisch/servo,jimberlage/servo,indykish/servo,AnthonyBroadCrawford/servo,saratang/servo,DominoTree/servo,dati91/servo,fiji-flo/servo,SimonSapin/servo,mattnenterprise/servo,fiji-flo/servo,splav/servo,dati91/servo,eddyb/servo,wpgallih/servo,anthgur/servo,cbrewster/servo,froydnj/servo,huonw/servo,shrenikgala/servo,karlito40/servo,jlegendary/servo,CJ8664/servo,Shraddha5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