Only 1 popover or dropdown should be visible at a time

Created by: sqs

Repro:

1. Visit any page with a repo header, such as https://sourcegraph.com/github.com/sourcegraph/docsite
2. Click the rev name (master) to open the rev popover
3. Click the down caret next to the repo name to open the repo popover

Expected: at most 1 popover should be able to be displayed at a given time. Clicking the repo name caret should close the rev popover.

Actual: both popovers are shown