R1:b584834b1934

In Differential: when the file tree is enabled, default to the "History" tab instead of "Files"

Summary:
Ref T13187. See PHI811. If the file tree is enabled and visible, set the default tab to "History".

- This is a bit magic.
- It won't work entirely on mobile (we can't tell that you're on mobile on the server, so we'll pick the "History" tab even though the file tree isn't actually visible on your device).
- There's no corresponding logic in Diffusion. Diffusion doesn't have the same tab layout…