Without `--no-track`, some git setups may automatically set
`origin/master` as the tracked upstream for the newly created branch.
This upstream is what hub defaults as --head when opening PRs. By not
allowing git to set `origin/master` as upstream, hub can then use the
proper --head for the PR.

Since hub still needs to know what --head is intended to be, we can set
the branch's upstream when pushing: with the --set-upstream option.

Fixes #755