R1:741118a08f3d

Improve Diffusion behavior for directories with impressive numbers of files

Summary:
Fixes T4366. Two years ago, Facebook put 16,000 files in a directory. Today, the page has nearly loaded.

Paginate large directories.

Test Plan:
- Viewed home and browse views in Git, Mercurial and Subversion.

I put an artificially small page size (5) on home:

{F1055653}

I pushed 16,000 files to a directory and paged through them. Here's the last page, which rendered in about 200ms:

{F1055655}

Our behavior is a…