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.

src/applications/diffusion/query/rawdiff/DiffusionGitRawDiffQuery.php

@@ -23,7 +23,7 @@ final class DiffusionGitRawDiffQuery extends DiffusionRawDiffQuery {
       // Check if this is the root commit by seeing if it has parents, since
       // `git diff X^ X` does not work if "X" is the initial commit.
       list($parents) = $repository->execxLocalCommand(
-        'log --format=%s %s --',
+        'log -n 1 --format=%s %s --',
         '%P',
         $commit);