Prefer Mercurial and Git over Subversion in discovering repository

Summary: svn info is the slowest command for discovering repository (up to 300 ms) and Subversion is probably the least used repository type with Arcanist. Let's discover it last.

Test Plan:

$ arc lint --trace

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4039