fix: again allow formatting the working copy while merging

I forgot about unfinished merges (I think I was thinking only about
unfinished merge conflicts) when I wrote
https://phab.mercurial-scm.org/D11209. As a coworker (hg contributor
dploch) reported to me, this led to hg fix --working-dir failing
when you have an uncommitted merge. The fix is trivial: just move the
assertion to just before the call to scmutil.movedirstate() where it
actually matters.

Differential Revision: https://phab.mercurial-scm.org/D11376