Merge branch 'rs-issue-1887' into 'master'

Reorder MergeRequestTabs constructor

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

See merge request !892