Make PhutilGitURI and PhutilURI strict about leading whitespace

Summary: Fixes T4913. We silently accept git and normal URIs with leading spaces, but other systems will not accept these. Instead, fail to parse these so they don't get into the system if we're validating URIs.

Test Plan: {F148680}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4913

Differential Revision: https://secure.phabricator.com/D8897