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