The --tracking-branch option is required for determining the upstream

branch for rbt land, but it was not passed in due to it being in the

diff_options group. It is now in its own option group,

branch_options, which is passed to all commands that accepted

diff_options as well as rbt land. This fixes an issue where

repositories that don't use the default (e.g., origin/master for git)

tracking branch would try to push to a non-existant branch.

With this patch applied, I am able to do rbt land --push when the

upstream branch is not origin/master.