exchange: support declaring pull depth Upcoming commits will teach exchangev2 how to perform a shallow clone. This commit teaches hg.clone(), exchange.pull(), and exchange.pulloperation to recognize a request for a shallow clone by having the caller specify a numeric depth of the maximum number of ancestor changesets to fetch. There are certainly other ways we could control shallow-ness. But this one is simple to implement and is also how the narrow extension controls things. So it seems to make sense to start here. Differential Revision: https://phab.mercurial-scm.org/D5136