Make discovery slightly cheaper in the common case

Summary:
Ref T4605. Before discovering branches, try to prefill the cache in bulk. For repositories with large numbers of branches, this allows us to issue dramatically fewer queries.

(Before D8780, this cache was usually held across discovery events, so being able to fill it cheaply was not as relevant.)

Test Plan: Ran discovery on Git, Mercurial and SVN repositories. Observed fewer queries for Git/Mercurial.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4605

Differential Revision: https://secure.phabricator.com/D8781