No-arguments rebase proceeds even if there's no upstream set

[zombiezen]

Steps to Reproduce

Run gg rebase with no arguments on a branch without an upstream.

Expected Behavior

gg should exit with an error message and otherwise do nothing.

Actual Behavior

gg appears to try the rebase.

Version Info

gg version 1.0.2-3, built on 2020-09-05T20:57:10Z
go: go1.14.7 gc linux/amd64
git version 2.20.1