Fix issue in Mercurial repos with duplicate branch heads

Summary:
Fixes T5613. A branch may have multiple heads in Mercurial, but `executeOne()` expects exactly one result.

Load them all instead. Equivalently, we could `limit(1)`, but it's likely that we'll use the cursors in the future to reduce the number of VCS operations we do, so this is probably a little more along the lines where we're headed.

Test Plan: Poked around some repos.

Reviewers: chad, richardvanvelzen

Reviewed By: richardvanvelzen

Subscribers: epriestley

Maniphest Tasks: T5613

Differential Revision: https://secure.phabricator.com/D9918