Detect missing 'svn', 'hg' and 'git' during setup

Summary:
These are a bit tricky because we don't want to require you to install a VCS you don't use just to use Phabricator. Test that repositories exist before performing the checks.

I'll couple this with additional checks during repository creation.

Test Plan: Changed binary names to nonexistent ones, verified setup issues raised properly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6040