Use ancestors(x) instead of 0::x in Mercurial history queries

Summary: If 0 isn't an ancestor of the current branch, the 0::x construction fails. This is uncommon, but not wildly unreasonable. The ancestors() construction is simpler anyway.

Test Plan: Viewed some hg repos locally (change history, file history) without anything suspicious cropping up.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7844