Trying to validate the fulltext of an external revision causes missing blobs to
be downloaded and cached. Since the downloads aren't batch prefetched and
aren't compressed, this can be expensive both in terms of time and space.
I made this a tri-state instead of a simple bool because there's an existing
(undocumented) config to handle this, and it would be weird if hg verify were
to suddenly start ignoring that config but an hg recover initiated verify
honors it. Since this uses the same config setting, it too will skip
rename verification (which requires fulltext, but not for LFS).