convert: also catch missing revlogs when introduced in repo roots
The previous behaviour was almost as if convert.hg.ignoreerrors was always set
for revisions without parents, except that errors were silently ignored. Revlog
errors are handled as a side effect of getcopies(), but getcopies() was only
called when convert.hg.ignoreerrors was set.
Now we always call self.getcopies for root revisions, not only when
convert.hg.ignoreerrors is set, just like we do on all other revisions.
The extra call might be a bit expensive, but the proper fix for that would be
to catch these errors in another way.