If repository has multiple remotes with the same name of branch
checking out to the branch using simple `git checkout branch` exists
with an error: this is because there is ambiguity w.r.t which remote
to track.
Now the code explicitly provides remote to track, preferring to use
"origin" first (which is to be used for Blender style of workflow,
and Github style workflow when there is a fork available), and use
"upstream" if there is no origin.
Sergey Sharybin