R1:d112b6c15d8e

Add `diffusion.querycommits` and deprecate `diffusion.getcommits`

Summary:
Fixes T4344. `diffusion.getcommits` is nasty old bad news. Implement a modern query method.

This method provides limit/paging in a somewhat abstract way so it's sort of ultramodern, but I didn't want the default behavior to return a million rows. I'll probably move more stuff toward this over time, now that cursor paging is pervasive. Here, we needed extra metadata (the identifier map) anyway.

Test Plan: Used console to execute…