Reject SCP-style URIs with explicit protocols in Diffusion

Summary:
Fixes T3619. These URIs are valid:

git@domain.com:/path        (Git SCP-style implicit SSH)
ssh://git@domain.com/path   (Explicit SSH)

This URI, arrived at by adding "ssh://" to the front of an SCP-style URI, is not:

ssh://git@domain.com:/path

Detect URIs in this form and reject them. See T3619.

Test Plan:
{F75486}

Also set some valid URIs.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3619

Differential Revision: https://secure.phabricator.com/D7431