Merge PhutilGitURI into PhutilURI

Summary:
Ref T11137. This addresses three general issues:

• From T11137, I recently added a bad typehint.
• From T11004, we currently have buggy behavior with Git URIs that have relative paths: we convert them into "ssh://" URIs but change the meaning of their paths.
• Generally, Phabricator has a lot of instanceof-flavored code around URI handling, and this has been a frequent source of errors. See some discussion in T11137. See next revision for changes to Phabricator.

Test Plan: Unit tests; see next revision for field testing.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11137

Differential Revision: https://secure.phabricator.com/D16099