convert: preserve highwater mark from v1 msgmap

convert: preserve highwater mark from v1 msgmap

If we're reusing the msgmap from a v1 inbox, we also need to
ensure the highwater mark doesn't get doubled in the v1->v2
conversion by internally triggering the equivalent of
"--reindex" on a fresh v2 inbox.

This was needed to convert an indexed v1 inbox which featured
messages with multiple Message-IDs in it.  Fresh, unindexed
clones of v1 inboxes would not have been affected by this.