tests: expand flags to long form in test-largefiles-update.t
I spent some time confused by this test. I'm pretty sure that this
line intends to be cleaning the dirstate, not checking that it's clean
before updating: the preceding #if block leaves the dirstate clean in
the noexec case, and dirty in the exec case, so we can't expect
consistent behavior across that platform variation. A subsequent patch
will modify this command to use --clean instead of --check.
I'll elaborate in that patch about the hypothetical bug here.