gofix: fix diff regression from exec change
gofix: fix diff regression from exec change

Also pass -u to diff to be consistent with gofmt.

Fixes issue 1619

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/4591041

webfd/go-zh,bryanxu/go-zh,Triskite/willstone-goclone,rdp/rogerpack2005-golang,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,mhennings/marcohennings-go,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,d0f/go-zh,scirelli/scirelli-go,npe9/noahevans-go9,webfd/go-zh,npe9/noahevans-go9,npe9/noahevans-go9,npe9/noahevans-go9,webfd/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,bryanxu/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,Triskite/willstone-goclone,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,npe9/noahevans-go9,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,npe9/noahevans-go9,sanjosh/sanjos100-tipc,scirelli/scirelli-go,Triskite/willstone-goclone,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,npe9/noahevans-go9,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,rdp/rogerpack2005-golang,Triskite/willstone-goclone,rdp/rogerpack2005-golang,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,Triskite/willstone-goclone,mhennings/marcohennings-go,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh