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 8ms instead of 200ms executing this
command, in a very small repository.