R1:2a355d8548e7

Make Diffusion "Last Modified" rendering less fragile

Summary:
Ref T9524. Because fetching the last time files were modified in Diffusion can be slow, we bring it in over Ajax.

The logic to fetch and paint the table is kind of fragile because there are two different definitions of the columns right now and we break in a bad way if they differ.

In particular, calling `diffusion.updatecoverage` can populate a "lint commit" for a repository, which tries to generate lint information in one of the views (but…