Ignore unrecognized refs in "refs/remotes/"

Summary: Ref T9028. When selecting refs, pretend refs in "refs/remotes/" that we don't otherwise recognize don't exist, since it looks like these are probably remotes of the remote we're observing, and who knows what state they're in.

Test Plan: Used bin/repository discover --verbose to verify that these named refs no longer appear in the list.

Reviewers: chad, joshuaspence

Reviewed By: joshuaspence

Maniphest Tasks: T9028

Differential Revision: https://secure.phabricator.com/D16136