tests: correct (I think) command in test-largefiles-update

When this test was introduced, it used the short-form of all the flags
on this update invocation. I suspect, based on the "start with clean
dirstates" comment and the fact that the no-exec branch of the #if
guard leaves dirstate clean, that this should have been 'update -qCr'
instead of 'update -qcr', but that a bug in largefiles --check
handling left this problem unnoticed.

I'll leave a breadcrumb further up about the current failure mode in
the hopes that we can fix this some day.

This was previously discussed in [0] but the trail in that thread goes
cold after a few replies. Given that this is still a flaky test, that
appears to only be passing by bad fortune, I think it's worth
correcting the code of the test to make a correct assertion, and to
keep track of the suspected bug with some other mechanism than an
invalid test (if we had support for "expected failure" blocks this
might be a worthwhile use of them?).

0: https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-October/089501.html

tests/test-largefiles-update.t

@@ -737,7 +737,7 b' files correctly after an interrupted upd'
 hashes reveals it isn't clean.
 
 Start with clean dirstates:
-  $ hg up --quiet --check --rev "8^"
+  $ hg up --quiet --clean --rev "8^"
   $ sleep 1
   $ hg st
 Update standins without updating largefiles - large1 is modified and largeX is