R1:ce632d64906b

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