Disambiguate "host.com:123" as a port, not a relative path

Summary:
Ref T11222. When we parse "host.com:123", it could be a Git-style URI pointing at the file named "123" on the host. We currently interpret it as such.

Instead, always assume that completely numeric "paths" are port numbers.

Test Plan: Added failing test; made it pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11222

Differential Revision: https://secure.phabricator.com/D16187