SelectedCommit is context-dependent and points to SelectedLocalCommit,
SelectedReflogCommit, or SelectedSubCommit depending on which panel is active.
If none of these panels is active, it returns the selected local commit, which
is probably the most useful default (e.g. when defining custom commands for the
Files panel).
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.
This guards against accidentally renaming a model field and thereby breaking
user's custom commands. With this change we'll get a build failure when we do
that.
Sometimes it takes a while to get PRs accepted upstream, and this blocks our
progress. Since I'm pretty much the only one making changes there anyway, it
makes sense to point to my fork directly.
