R1:a74545c9da4b

Provide a rough, unstable API for reporting coverage into Diffusion

Summary:
Ref T4994. This stuff works:

- You can dump a blob of coverage information into `diffusion.updatecoverage`. This wipes existing coverage information and replaces it.
- It shows up when viewing files.
- It shows up when viewing commits.

This stuff does not work:

- When viewing files, the Javascript hover interaction isn't tied in yet.
- We always show this information, even if you're behind the commit where it was…