verify: add a hook that can let extensions manipulate file lists
Without a hook of this nature, narrowhg[0] clones always result in 'hg
verify' reporting terrible damage to the entire repository
history. With this hook, we can ignore files that aren't supposed to
be in the clone, and then get an accurate report of any damage present
(or not) in the repo.
0: https://bitbucket.org/Google/narrowhg