Increase strictness of URI parsing, rejecting URIs in the form "ssh://-flag"

Summary:
Ref T12961. See that task for discussion of the major attack we're responding to here.

• Reject hosts beginning with "-". These are not legitimate.
• Reject hosts beginning with ".". These are also not legitimate.
• Tighten $ to \z. $ can match either "newline, end of string" or "end of string". \z matches ONLY "end of string". We don't want to match a newline, only "end of string" strictly.
• We already that hosts otherwise contain only "reasonable" characters (letters, numbers, hyphens, and periods).

Test Plan:

• Added unit tests, ran unit tests.
• Tried to set a repository URI to ssh://-oxyz/path with these changes, which worked previously; it no longer works.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12961

Differential Revision: https://secure.phabricator.com/D18388