R1:9329e6a12d9f

Stop doing an excessive amount of work in `diffusion.rawdiffquery`

Ref T11665.

Without `-n 1`, this logs the ENTIRE history of the repository. We
actually get the right result, but this is egregiously slow. Add `-n 1`
to return only one result.

It appears that I wrote this wrong way back in 2011, in D953. This
query is rarely used (until recently) which is likely why it has
escaped notice for so long.

Test Plan: Used Conduit console to execute `diffusion.rawdiffquery`.
Got the same results but spent…