Make discovery slightly cheaper in the common case
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.
Reviewed By: btrahan
Maniphest Tasks: T4605
Differential Revision: https://secure.phabricator.com/D8781