Check that the current HEAD is reasonable when doing a push

[tgross35]

I accidentally started a push from a main that wasn't the upstream main, and just barely realized because I clicked the link in the PR description and got the "This commit does not belong to any branch on this repository" warning. Maybe we could check that the current local commit exists on the remote's default branch, and error if not?

[tgross35]

A warning that the commit isn't the latest may also be nice if the local is behind the remote.