feat(git): group push menu by branch target

[markwylde]

Summary

• group the Git sidebar push menu under current, new, and default branch targets
• resolve the repository default branch independently from the active worktree branch
• add quick-push support for pushing directly to the default branch

Verification

• npm run lint
• npm run build:app
• npm run test:quick-push-pr
• npm run test:e2e -- e2e/file-explorer-sidebar.spec.ts --grep "git sidebar pane renders a nested tree and offers a push menu"