revlog: add a method to retrieve snapshot depth
Some snapshot property (eg: maximum size) will depend on their depth.
https://bz.mercurial-scm.org/1089 $ hg init $ mkdir a $ echo a > a/b $ hg ci -Am m adding a/b $ hg rm a removing a/b $ hg ci -m m a $ mkdir a b $ echo a > a/b $ hg ci -Am m adding a/b $ hg rm a removing a/b $ cd bRelative delete: $ hg ci -m m ../a $ cd ..