Add PhutilGitURI for parsing scp-style Git URIs

Summary:
See T1529.

We currently convert "scp-style" URIs into normal URIs in Phabricator:

git@host:path -> ssh://git@host/path

This isn't correct; paths in the scp-style URIs are resolved relatively, while paths in the normal-style URIs are resolved absolutely.

Since we can't do this transformation any more, add a real parser so we can replace the transformation code in Phabricator with something sensible.

Test Plan: Unit test coverage.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1529

Differential Revision: https://secure.phabricator.com/D3033