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.

Used console to execute command.