lfs: add a switch to `hg verify` to ignore the content of blobs Trying to validate the fulltext of an external revision causes missing blobs to be downloaded and cached. Since the downloads aren't batch prefetched[1] 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). [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-April/116118.html Differential Revision: https://phab.mercurial-scm.org/D7708