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 command.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4344

Differential Revision: https://secure.phabricator.com/D8077