In a triangular workflow the branch that you're pulling from is not the same as
the one that you are pushing to. For example, some people find it useful to set
the upstream branch to origin/master so that pulling effectively rebases onto
master, and set the push.default git config to "current" so that "feature"
pushes to origin/feature.
Another example is a fork-based workflow where "feature" has upstream set to
upstream/main, and the repo has remote.pushDefault set to "origin", so pushing
on "feature" pushes to origin/feature.
This commit adds new fields to models.Branch that store the ahead/behind
information against the push branch; for the "normal" workflow where you pull
and push from/to the upstream branch, AheadForPush/BehindForPush will be the
same as AheadForPull/BehindForPull.
