Fix scm_build_file in symlinked directories
Fix scm_build_file in symlinked directories

Use the realpath of self.root_dir, because otherwise repos in
symlinked directories (e.g the tests when run on OS X) will not work.

Additionally, move the full_path computation above some error messages
that use it.

Testing Done:
Ran scm_build_file test on OS X.

Reviewed at https://rbcommons.com/s/twitter/r/2152/

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