Fix a build invalidator bug.
Fix a build invalidator bug.

A target's cache key contains the representation of its jar deps.
However the new Artifact class had no __repr__, so that representation
was Python's default one, which contains the memory address of the
object... Since this is likely to change from run to run, the result
is a ton of spurious invalidation.

(sapling split of 40a51bd134df05ece1d5a585b23e4a4758a31de2)
pombredanne/pants,UnrememberMe/pants,Gabriel439/pants,kslundberg/pants,dgomez10/xanon,benjyw/pants,pgroudas/pants,wisechengyi/pants,jtrobec/pants,qma/pants,peiyuwang/pants,laurentgo/pants,pantsbuild/pants,TansyArron/pants,slyphon/pants,TansyArron/pants,ericzundel/pants,tdyas/pants,ity/pants,areitz/pants,dgomez10/xanon,baroquebobcat/pants,dturner-tw/pants,tejal29/pants,gmalmquist/pants,mateor/pants,mateor/pants,UnrememberMe/pants,manasapte/pants,slyphon/pants,ericzundel/pants,landism/pants,wisechengyi/pants,dgomez10/xanon,laurentgo/pants,sid-kap/pants,ericzundel/pants,cevaris/pants,kslundberg/pants,fkorotkov/pants,dgomez10/xanon,benjyw/pants,tdyas/pants,twitter/pants,foursquare/pants,dgomez10/xanon,scode/pants,fkorotkov/pants,baroquebobcat/pants,peiyuwang/pants,manasapte/pants,UnrememberMe/pants,areitz/pants,dbentley/pants,tejal29/pants,UnrememberMe/pants,kwlzn/pants,wisechengyi/pants,foursquare/pants,gmalmquist/pants,megaserg/pants,digwanderlust/pants,dbentley/pants,digwanderlust/pants,ericzundel/pants,kwlzn/pants,peiyuwang/pants,jessrosenfield/pants,lahosken/pants,baroquebobcat/pants,mateor/pants,sid-kap/pants,dturner-tw/pants,Gabriel439/pants,cevaris/pants,pombredanne/pants,manasapte/pants,wisechengyi/pants,scode/pants,square/pants,areitz/pants,megaserg/pants,sameerparekh/pants,dbentley/pants,manasapte/pants,Gabriel439/pants,ity/pants,manasapte/pants,digwanderlust/pants,twitter/pants,wisechengyi/pants,lahosken/pants,pantsbuild/pants,qma/pants,foursquare/pants,TansyArron/pants,dbentley/pants,kwlzn/pants,dturner-tw/pants,benjyw/pants,baroquebobcat/pants,di0spyr0s/pants,mateor/pants,twitter/pants,Gabriel439/pants,scode/pants,lahosken/pants,lahosken/pants,kslundberg/pants,dgomez10/xanon,peiyuwang/pants,tejal29/pants,foursquare/pants,baroquebobcat/pants,UnrememberMe/pants,di0spyr0s/pants,benjyw/pants,ericzundel/pants,UnrememberMe/pants,benjyw/pants,mateor/pants,pantsbuild/pants,peiyuwang/pants,fkorotkov/pants,tejal29/pants,qma/pants,fkorotkov/pants,laurentgo/pants,twitter/pants,twitter/pants,areitz/pants,jsirois/pants,jessrosenfield/pants,tejal29/pants,twitter/pants,dbentley/pants,ericzundel/pants,twitter/pants,mateor/pants,scode/pants,benjyw/pants,gmalmquist/pants,landism/pants,jsirois/pants,jessrosenfield/pants,15Dkatz/pants,baroquebobcat/pants,jtrobec/pants,ity/pants,scode/pants,jessrosenfield/pants,foursquare/pants,scode/pants,pantsbuild/pants,dbentley/pants,TansyArron/pants,landism/pants,baroquebobcat/pants,wisechengyi/pants,TansyArron/pants,peiyuwang/pants,fkorotkov/pants,jessrosenfield/pants,gmalmquist/pants,megaserg/pants,UnrememberMe/pants,sameerparekh/pants,jtrobec/pants,sid-kap/pants,dgomez10/xanon,laurentgo/pants,dgomez10/xanon,landism/pants,slyphon/pants,UnrememberMe/pants,gmalmquist/pants,15Dkatz/pants,kslundberg/pants,jtrobec/pants,lahosken/pants,megaserg/pants,tdyas/pants,dturner-tw/pants,areitz/pants,pantsbuild/pants,Gabriel439/pants,qma/pants,TansyArron/pants,foursquare/pants,megaserg/pants,cevaris/pants,di0spyr0s/pants,UnrememberMe/pants,sid-kap/pants,kwlzn/pants,wisechengyi/pants,cevaris/pants,dgomez10/xanon,landism/pants,pantsbuild/pants,peiyuwang/pants,kwlzn/pants,baroquebobcat/pants,square/pants,jtrobec/pants,fkorotkov/pants,kwlzn/pants,peiyuwang/pants,laurentgo/pants,foursquare/pants,baroquebobcat/pants,pgroudas/pants,Gabriel439/pants,tdyas/pants,landism/pants,15Dkatz/pants,tdyas/pants,pgroudas/pants,15Dkatz/pants,landism/pants,qma/pants,dturner-tw/pants,pombredanne/pants,benjyw/pants,kslundberg/pants,cevaris/pants,digwanderlust/pants,jtrobec/pants,di0spyr0s/pants,15Dkatz/pants,15Dkatz/pants,qma/pants,slyphon/pants,tdyas/pants,pombredanne/pants,sid-kap/pants,cevaris/pants,dturner-tw/pants,ericzundel/pants,pantsbuild/pants,pgroudas/pants,kslundberg/pants,jessrosenfield/pants,pgroudas/pants,tdyas/pants,fkorotkov/pants,tdyas/pants,jessrosenfield/pants,kslundberg/pants,sid-kap/pants,pombredanne/pants,twitter/pants,gmalmquist/pants,areitz/pants,scode/pants,ity/pants,digwanderlust/pants,slyphon/pants,15Dkatz/pants,ericzundel/pants,megaserg/pants,lahosken/pants,mateor/pants,square/pants,TansyArron/pants,twitter/pants,di0spyr0s/pants,fkorotkov/pants,ity/pants,jsirois/pants,mateor/pants,lahosken/pants,15Dkatz/pants,tdyas/pants,areitz/pants,megaserg/pants,pombredanne/pants,sid-kap/pants,wisechengyi/pants,ity/pants,pombredanne/pants,pgroudas/pants,square/pants,slyphon/pants,sameerparekh/pants,ity/pants,sameerparekh/pants,kwlzn/pants,di0spyr0s/pants,wisechengyi/pants,dturner-tw/pants,qma/pants,laurentgo/pants,sameerparekh/pants,cevaris/pants,dgomez10/xanon,foursquare/pants,sameerparekh/pants,manasapte/pants,digwanderlust/pants,landism/pants,slyphon/pants,dgomez10/xanon,lahosken/pants,jtrobec/pants,manasapte/pants,di0spyr0s/pants,Gabriel439/pants,foursquare/pants,dbentley/pants,gmalmquist/pants,sameerparekh/pants