When updating git repositories, use --prune to prune old branches

Summary: We'll keep deleted branches around right now because Git's behavior is not to remove them without --prune.

Test Plan: Ran "git fetch --all --prune" to make sure it at least ostensibly works.

Reviewers: btrahan, 20after4

Reviewed By: 20after4

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1833