Fix add_target to walk the added target and thus find all its internal and external dependencies lke we do for the root set.
Fix add_target to walk the added target and thus find all its internal and external dependencies lke we do for the root set.

(sapling split of 709391881b82b98329aa5a0e00519c0aa57a0ab1)
slyphon/pants,dbentley/pants,lahosken/pants,peiyuwang/pants,di0spyr0s/pants,foursquare/pants,TansyArron/pants,areitz/pants,foursquare/pants,tdyas/pants,wisechengyi/pants,wisechengyi/pants,Gabriel439/pants,digwanderlust/pants,UnrememberMe/pants,pombredanne/pants,twitter/pants,kslundberg/pants,gmalmquist/pants,digwanderlust/pants,fkorotkov/pants,ericzundel/pants,manasapte/pants,mateor/pants,di0spyr0s/pants,qma/pants,dgomez10/xanon,foursquare/pants,sid-kap/pants,ity/pants,dgomez10/xanon,peiyuwang/pants,dturner-tw/pants,dgomez10/xanon,pantsbuild/pants,baroquebobcat/pants,twitter/pants,pombredanne/pants,di0spyr0s/pants,sid-kap/pants,pgroudas/pants,tdyas/pants,slyphon/pants,kslundberg/pants,peiyuwang/pants,ity/pants,ericzundel/pants,tdyas/pants,tdyas/pants,pgroudas/pants,wisechengyi/pants,benjyw/pants,square/pants,cevaris/pants,gmalmquist/pants,jsirois/pants,laurentgo/pants,kslundberg/pants,laurentgo/pants,TansyArron/pants,15Dkatz/pants,scode/pants,di0spyr0s/pants,twitter/pants,landism/pants,areitz/pants,kwlzn/pants,manasapte/pants,twitter/pants,twitter/pants,kwlzn/pants,landism/pants,sameerparekh/pants,pantsbuild/pants,15Dkatz/pants,dturner-tw/pants,dgomez10/xanon,square/pants,TansyArron/pants,peiyuwang/pants,sameerparekh/pants,pgroudas/pants,sid-kap/pants,twitter/pants,pgroudas/pants,areitz/pants,dbentley/pants,ity/pants,fkorotkov/pants,benjyw/pants,qma/pants,manasapte/pants,pombredanne/pants,tdyas/pants,lahosken/pants,megaserg/pants,sameerparekh/pants,jessrosenfield/pants,ericzundel/pants,jessrosenfield/pants,manasapte/pants,foursquare/pants,tdyas/pants,dgomez10/xanon,slyphon/pants,dbentley/pants,benjyw/pants,dbentley/pants,sid-kap/pants,twitter/pants,baroquebobcat/pants,square/pants,mateor/pants,cevaris/pants,wisechengyi/pants,slyphon/pants,areitz/pants,dbentley/pants,Gabriel439/pants,dbentley/pants,qma/pants,ericzundel/pants,manasapte/pants,15Dkatz/pants,peiyuwang/pants,pantsbuild/pants,scode/pants,TansyArron/pants,jessrosenfield/pants,kslundberg/pants,megaserg/pants,pantsbuild/pants,kwlzn/pants,jtrobec/pants,gmalmquist/pants,kslundberg/pants,digwanderlust/pants,mateor/pants,wisechengyi/pants,dturner-tw/pants,jtrobec/pants,scode/pants,cevaris/pants,dbentley/pants,fkorotkov/pants,manasapte/pants,jtrobec/pants,qma/pants,cevaris/pants,fkorotkov/pants,jessrosenfield/pants,dturner-tw/pants,kwlzn/pants,pombredanne/pants,dturner-tw/pants,di0spyr0s/pants,mateor/pants,ericzundel/pants,digwanderlust/pants,dgomez10/xanon,benjyw/pants,wisechengyi/pants,manasapte/pants,tejal29/pants,UnrememberMe/pants,kwlzn/pants,laurentgo/pants,wisechengyi/pants,areitz/pants,tejal29/pants,qma/pants,scode/pants,ity/pants,gmalmquist/pants,UnrememberMe/pants,tdyas/pants,gmalmquist/pants,digwanderlust/pants,areitz/pants,pombredanne/pants,Gabriel439/pants,foursquare/pants,landism/pants,lahosken/pants,jessrosenfield/pants,baroquebobcat/pants,pantsbuild/pants,UnrememberMe/pants,laurentgo/pants,lahosken/pants,baroquebobcat/pants,qma/pants,di0spyr0s/pants,jtrobec/pants,cevaris/pants,sameerparekh/pants,TansyArron/pants,dturner-tw/pants,UnrememberMe/pants,scode/pants,baroquebobcat/pants,TansyArron/pants,15Dkatz/pants,peiyuwang/pants,landism/pants,megaserg/pants,fkorotkov/pants,dgomez10/xanon,lahosken/pants,foursquare/pants,laurentgo/pants,tejal29/pants,peiyuwang/pants,jsirois/pants,baroquebobcat/pants,tdyas/pants,tejal29/pants,ericzundel/pants,dgomez10/xanon,tejal29/pants,laurentgo/pants,UnrememberMe/pants,benjyw/pants,foursquare/pants,sid-kap/pants,sid-kap/pants,pantsbuild/pants,kslundberg/pants,landism/pants,pombredanne/pants,fkorotkov/pants,dgomez10/xanon,15Dkatz/pants,landism/pants,lahosken/pants,pombredanne/pants,benjyw/pants,pgroudas/pants,UnrememberMe/pants,wisechengyi/pants,15Dkatz/pants,kslundberg/pants,jtrobec/pants,megaserg/pants,dgomez10/xanon,slyphon/pants,cevaris/pants,lahosken/pants,benjyw/pants,15Dkatz/pants,ericzundel/pants,mateor/pants,UnrememberMe/pants,peiyuwang/pants,cevaris/pants,fkorotkov/pants,scode/pants,wisechengyi/pants,Gabriel439/pants,digwanderlust/pants,qma/pants,twitter/pants,Gabriel439/pants,sid-kap/pants,baroquebobcat/pants,foursquare/pants,jtrobec/pants,jtrobec/pants,Gabriel439/pants,mateor/pants,15Dkatz/pants,mateor/pants,TansyArron/pants,square/pants,Gabriel439/pants,slyphon/pants,areitz/pants,kwlzn/pants,baroquebobcat/pants,ericzundel/pants,slyphon/pants,kwlzn/pants,baroquebobcat/pants,gmalmquist/pants,gmalmquist/pants,scode/pants,ity/pants,UnrememberMe/pants,megaserg/pants,landism/pants,di0spyr0s/pants,sameerparekh/pants,landism/pants,foursquare/pants,megaserg/pants,ity/pants,fkorotkov/pants,jessrosenfield/pants,jessrosenfield/pants,sameerparekh/pants,dturner-tw/pants,tdyas/pants,sameerparekh/pants,jsirois/pants,dgomez10/xanon,twitter/pants,lahosken/pants,megaserg/pants,pantsbuild/pants,pgroudas/pants,mateor/pants,ity/pants