Respect `gitsavvy.pushdefault` when pushing tags

[kaste]

When you have multiple remotes, GitSavvy asks where to push a new tag to.

• Honor pushdefault -- however that is actually set; need to look that up
• Likely, do a heuristic, i.e. just choose origin.

Then -- similar to how we have that for "push", just ask push to "origin/0.1.2 | configure where to push to". That way we have all the options we want but a better UX.