fix "show more" button not showing bug

Created by: slimsag

This fixes a P1 customer issue and release blocker https://github.com/sourcegraph/sourcegraph/issues/4564 so I'd like to get this merged ASAP.

Open questions:

1. Can this be tested somehow with sinon or something? It is unclear to me how I would do this because of the interaction with setState here.
2. This is clearly an example of code where rxjs would help avoid setState race conditions -- so I feel like the correct fix here would be refactoring this code to use rxjs or maybe maintaining a separate state for whether or not show more should be shown -- but I am uncertain about the best path forward with this. Help?

Test plan: