Improve performance of repository discovery under Mercurial

Summary: Prelminary, I want to test this a bit more when I'm better rested. Provide a "bulk" import mode for Mercurial so we can do initial discovery more quickly.

Test Plan: Imported the Mercurial repository in 2m45s, blocked on MySQL I/O rather than Mercurial I/O.

Reviewers: csilvers, btrahan

Reviewed By: csilvers

CC: aran

Differential Revision: https://secure.phabricator.com/D2457