Clearing it before the conversion protects us from whatever data were
there (file copies in particular).
Invalidating it after the conversion avoids writing a possibly
inconsistent dirstate to disk.
During a conversion, the dirstate contents are not consistent - there
are files that may be missing from the dirstate and there may be files
that shouldn't be in the dirstate.
While this is not fixed, don't mark files as added - put them directly
in state 'n'ormal.