On the first load, the merge request tabs would be requested in JSON format to render
the HTML. This would cause the "Reload with full diff" button to create a link to diff.json.
Closes#2041
The tab was being activated before we set the `[commits|diffs]Loaded`
variable, so even when the `/diffs` route is accessed directly, like
from the "Side-by-side" link, the tab was being loaded from its default
source.
Fixes#1887
