Git 1.7 compatibility for publish

[mgraham]

grb publish doesn't work on git 1.7.x.

For git 1.8.x the command for setting the upstream branch of a local branch is:
'"#{GIT} branch -u #{origin}/#{branch_name} #{branch_name}"',

But for git 1.7.x, the command is:
'"#{GIT} branch --set-upstream #{branch_name} #{origin}/#{branch_name}"',